Gomu Gomu no Mi/Gear 4 Techniques

Gear 4 ("Gear Fourth") is a transformation technique that uses the power of the Gomu Gomu no Mi and Busoshoku Haki. It has multiple variations that enhance some amount of the user's defense, speed, and power beyond their normal limits. It was first seen in Luffy's battle against Donquixote Doflamingo. It was developed by Luffy during his training on Rusukaina.

Overview


Luffy coats his arm in Busoshoku Haki before biting into his forearm. Similar to how he activates Gear 3, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half. If Luffy is stopped mid-blowing, the arm will slowly deflate.

When utilizing Gear 4, Luffy makes extensive usage of his Busoshoku Haki by coating his arms, legs and much of his torso in it to the point that they turn black from hardening. The Haki coating across his body resembles the wispy ends made from flames, giving it the appearance of tattoos, and he gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall. Similar to when using Gear 2, Luffy is continuously emitting steam in this form. Combined with Luffy's rubbery composition and his compressed muscle structure, Gear 4 both adds explosive power to Luffy's attacks and grants him enhanced defensive capabilities far beyond what he can demonstrate with or without Gear 2 or Gear 3. Depending on the situation, Luffy can adapt Gear 4 into different forms.

While Luffy is using Gear 4, his physical strength and speed are increased to the point where he is able to overwhelm a Warlord of the Sea, Donquixote Doflamingo, and the second strongest member of an Emperor's crew, Charlotte Katakuri, whereas before, Gear 2 's lack of strength and Gear 3 's lack of speed made them ineffective in the fight.

Unlike the previous forms, Gear 4 focuses on compression more than stretching in a manner similar to the Bane Bane no Mi. He can compress his arms, legs, and head. While the compression grants Luffy's moves overwhelming speed and power, the range of his attacks is severely decreased. For example, the Gomu Gomu no Bazooka is a technique that Luffy has proven he can execute over varying distances, but due to needing to actually compress his fists for the Leo Bazooka, it required him to be right in front of Doflamingo. When the latter unveils his Devil Fruit Awakening, Luffy expresses his annoyance at his difficulty getting close to the Warlord of the Sea. However, he can still deliver long-ranged attacks by stretching the part of the body he wants to attack with while retaining great power and speed, as his body is still greater in size and covered in Busoshoku Haki. Luffy also seems to have abandoned most of his sweeping attacks in this gear, such as Whip and Scythe, due to relying on compression rather than stretching.

Luffy shows a much more versatile side of the Gomu Gomu no Mi's powers by changing the direction of his attacks mid-strike, by bouncing off the air, to hit a moving opponent without losing momentum. According to Kaidou, due to the nature of rubber, such an application should be impossible.



According to Doflamingo, the power of Luffy's attacks is greatly magnified in this form. His durability is also enhanced: despite being hardened with Busoshoku Haki, his body is still rubbery, so physical blows (even ones enhanced with Busoshoku Haki) simply bounce off of him and/or used as a counterattack, depending on the form Luffy use.

Using Gear 4 has a number of drawbacks for Luffy. There is an unknown limit to how long he can keep Gear 4 active, as noted by both Luffy himself and Doflamingo. Once this limit is reached, Gear 4 automatically deactivates, leaving Luffy exhausted and barely able to move. He is also left unable to use Haki for ten minutes after using this technique as it quickly depletes his Haki reserves. The technique also greatly taxes Luffy's stamina, as using it just once leaves Luffy so physically drained in the aftermath that he can barely stand and fight, even after he has regained his Haki and mobility. Even afterward, Gear 4 leaves its marks on Luffy's body. Akin to Gear 2, using Gear 4 causes Luffy's metabolism to increase rapidly. In fact, after Luffy stuffed himself to a bloated size in his fight with Charlotte Cracker, using Gear 4 once made him burn off the massive amount he had eaten in a manner of minutes. Furthermore, using Gear 4 briefly for a second time in a row once rendered him unconscious for three days. Using Gear 4 also makes him more prone to falling asleep. As such this "Gear" is completely used as a last resort if Gear 2 and Gear 3 fail to affect his opponents. Due to these side effects, Luffy is left vulnerable immediately after the depletion of Gear 4, particularly against powerful opponents. In fact, Luffy has yet to face a canonical opponent that he could defeat with a single use of Gear 4 aside from the minions of the Beasts Pirates.

However, the drawbacks have shown signs of diminishing, especially after eating food. While retreating from his battle with Katakuri when his first attempt to use Gear 4 against him failed, Luffy could still run and escape the Mirro-World using Charlotte Brûlée even after his Gear 4 wore out. He was still fast enough to dodge a surprise attack from Amande, and he could still use his other attacks (though he could not imbue them with Haki) as well as flee from a food-crazed Big Mom. It is further lessened following his training in Udon, from initially using Seastone cuffs, in addition to later training from Hyogoro to advance his Busoshoku Haki. At the start of the Raid on Onigashima, Luffy used Gear 4 to sink a Beasts Pirates ship and did not seem to be worn out after returning to normal form. However, during that time, Luffy deactivated Gear 4 on his own before reaching the time limit. During the battle against Kaidou and Big Mom on Onigashima, Luffy still cannot stay in Gear 4 for a certain amount of time and he will be worn out once he reaches his limit.

It is shown that Luffy is able to transition between at least two of his known forms.

In the anime, the luster of the Busoshoku black parts and the Busoshoku tattoos take on a reddish tint. This would later be included in the manga with the Snakeman form on the cover of Volume 89. Furthermore, in the anime, Luffy seems to take up some kabuki-like mannerisms, like exaggerated head-rolling, and occasionally dragging out words. Luffy's voice also gets rough and a little deeper.

Boundman


Luffy's first Gear 4 form is called Boundman: Luffy's proportions become warped, and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. As a side-effect, he is incapable of standing still on the ground, and instead constantly bounds on the spot. This form was created in order to combat the many powerful and large animals on Rusukaina.

This form, much like Gear 2, seems to utilize the principles of Rokushiki. It possesses both high-speed movement reminiscent of Soru and the ability to kick off the air, similar to Geppo, cleverly adapted to work even better with his body's elasticity: Luffy not only bounds off the air, but allows attacks to bounce off his own body. His body is also now strong enough to defend against cutting attacks head-on.

After Boundman lands on the ground after using an attack, Luffy cannot seem to control his landing due to warped proportions his body takes and bounces like a ball in a comical fashion until he either loses momentum or he somehow regains control and starts "flying" again.

Though he has some heightened resistance to cutting attacks in this form, if an opponent using a bladed weapon has Busoshoku Haki of comparable or greater strength to Luffy's own, they will be able to cut through his coated arms with relative ease. Similarly, his near immunity to blunt force attacks is also not absolute. If an opponent has powerful enough Haki in conjunction with their physical strength, it's possible to negate the rubbery defense of Luffy's inflated body and injure him. Additionally, the defensive abilities of this form can still be overcome by an attack of devastating sheer power. When Luffy attacked Kaidou, the latter counterattacked with his kanabo. Instead of bouncing, Luffy was sent flying and rendered unconscious by a single hit. Also, attacks from this form were initially not strong enough to harm the person who possesses immense durability as shown by Kaidou, who received a barrage of powerful attacks from Luffy, and remained unfazed by the attacks.

Tankman
During his battle with Charlotte Cracker, Luffy revealed another form of Gear 4 known as Tankman (タンクマン). Currently, the base form of "Tankman" is unknown, since the only time it has been used was after Luffy ate Cracker's Biscuits to the point where he was completely full and used his "Manpuku Version" form.

Due to the increase in Luffy's metabolism Gear 4 causes, he is able to leave his Tankman form in a short time after using it. This is the case when Luffy was able to burn off the massive amount of biscuits he had eaten in his fight with Cracker shortly after he defeated the Sweet Commander.

In this form, Luffy's arms inflate slightly while most of the air appears to fill his torso. Luffy's durability rises sharply in this form, as he was capable of tanking and deflecting a piercing strike from Charlotte Cracker, whereas previously Cracker was able to cut deep into Luffy's arm when Luffy was in Boundman.

Snakeman


During his battle with Charlotte Katakuri, Luffy revealed another form of Gear 4 known as Snakeman. Unlike his other two Gear 4 forms, Luffy does not significantly expand, with his forearms, legs, and upper torso being only slightly enlarged and the rest of his body retaining its normal proportions. The spread of Haki is slightly more limited, as it does not appear to extend onto Luffy's shoulders and upper body. However, Luffy's hair becomes flame-shaped like his Haki and sticks up. Instead of steam, this form produces a gaseous substance appearing as a mix of steam and fire and like its other counterparts also seems to be draped over his shoulders like a scarf while in this form. His fists also flex differently, with the first knuckles being extended, similar to leopard fist.



In this form, Luffy gains a significant increase in speed and ability to use "Python" to change the direction of his attacks, the latter of which he also possesses in his Boundman form. However, Snakeman allows his attacks to continuously accelerate the longer they last, allowing Luffy not only to press an attack more fiercely on an opponent capable of dodging but also to increase the attack's speed and power until he finally lands the hit. It also gives him the opportunity to attack any area of an opponent with ease. This form focuses on dealing with agile opponents who specialize in dodging and requires Luffy's recently heightened proficiency in Kenbunshoku Haki in order to make his punches into semi-homing attacks.

This form also allows Luffy to jump decent heights, as shown when he leaped out of a chasm in Mirro-World, though its jumping ability compared to Boundman is unknown.

However, because of his body's lack of inflation, Luffy's defense is considerably worse compared to his Boundman and Tankman forms. When he was thrown into structures by Katakuri, he took the impact like he normally would, as opposed to bouncing as he does in Boundman. Additionally, he receives blunt attacks as he does normally rather than having them bounce off him.

Techniques

 * Kin'niku Fusen (筋肉風船): By coating his arm in Busoshoku Haki and biting into his forearm, Luffy is able to blow a large quantity of air into his body and inflate his muscular structure, with the air blown into his arm spreading out to the rest of his upper body. In this form, his durability is enhanced to the point where a Busoshoku Haki enhanced kick from Donquixote Doflamingo, a Warlord of the Sea, bounced back harmlessly, due to his body retaining its rubber properties despite being coated in Haki.




 * Python: Using a combination of Busoshoku Haki and his rubber body, Luffy is now able to redirect his punches during an attack instead of retracting it back, allowing himself to bypass the necessary build-up time between attacks, making his attacks into semi-homing attacks. Whilst in Boundman form the attack can continue stretching without losing momentum; in his Snakeman form, the attack goes a step further by continuously accelerating until Luffy retracts it. However, Kaidou noted that this ability to bend the trajectory of attacks should not be possible given the innate nature of rubber. The kanji for this attack can be read as "Orochi", likely nodding to the famed Yamata-no-Orochi.

Boundman



 * Gomu Gomu no Kong Gun (ゴムゴムの): The Gear 4 version of Gomu Gomu no Bullet. After compressing his fist into his enlarged forearm (and making it look like the barrel of an actual cannon), Luffy flies towards his opponent at high speeds and unleashes a devastating short-range punch with massive power. This technique had enough strength to break through Doflamingo's own Haki-enhanced defense and hurl the Warlord all the way from Dressrosa's palace to the middle of the city. This technique is drastically more powerful than anything Luffy has utilized in Gear 2 or Gear 3, best demonstrated when he was completely unable to harm the 860,000,000 bounty Cracker with Hawk Gatling or Elephant Gun, having them blocked and deflected by his shields, but sent him reeling with a Kong Gun while also easily shattering the shields. Later, this technique also had enough strength to break through Katakuri's own Haki-enhanced defenses and hurl the Sweet Commander away when they fought in the Mirro-World. However, it was easily blocked by Charlotte Linlin when she hardened her elbow as well. After improving his Haki, Luffy was able to damage Kaidou. "Kong" refers to King Kong, the titular gigantic gorilla from the film of the same name. In the VIZ manga and Funimation adaptations, this is called Gum-Gum Kong Gun. The kanji's reading of "Enōjū" likely nods to the Monkey King, Sun Wukong/Son Gokū.


 * Gomu Gomu no Rhino Schneider (ゴムゴムの): The Gear 4 version of Gomu Gomu no Yari. After compressing both legs into his body, Luffy delivers a double flying kick. It has enough power to send the opponent crashing through dozens of buildings and across a town center. This was first used as a surprise attack against Doflamingo. After Luffy's training at Udon, this attack managed to injure Kaidou. A howitzer is a type of short-barreled artillery cannon, and "Schneider" refers to the Canon de 155 C modèle 1917 Schneider, a specific howitzer used by several countries during World War I. In the VIZ manga and Funimation adaptations, this is called Gum-Gum Rhino Schneider.


 * Gomu Gomu no Culverin (ゴムゴムの): The Gear 4 version of Gomu Gomu no Pistol, Luffy launches a regular straight punch at his opponent, augmented with stretching. However, if the attack misses, it can be combined with the Python technique to continue chasing after the opponent until it lands. It was first used while airborne against Doflamingo in Dressrosa and was powerful enough to send him plummeting to the ground at high speeds. A culverin is a relatively simple firearm, ancestor of the musket, and cannon. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Culverin. The "great serpent" portion of the kanji can also be read as "orochi".


 * Gomu Gomu no Double Culverin (ゴムゴムのダブル): The Gear 4 version of Gomu Gomu no Twin Pistol. Luffy pulls back his arms before punching his opponent with them. This was first used against Doflamingo, but was countered by his Off White. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Double Culverin.




 * Gomu Gomu no Leo Bazooka (ゴムゴムの・バズーカ): The Gear 4 version of Gomu Gomu no Bazooka. After retracting both of his fists into his arms, Luffy "fires" them at his opponent with a double open-palm strike. This was first used while airborne against Doflamingo and its power sent him flying over Dressrosa and caused him to create an enormous crater when he slammed into the side of the King's Plateau. Leo is Latin for "lion". In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Leo Bazooka.




 * Gomu Gomu no King Kong Gun (ゴムゴムの): The Gear 4 version of Gomu Gomu no Gigant Pistol and Gomu Gomu no Elephant Gun. After Luffy compresses his fist, as he does for Kong Gun, he blows a large amount of air into his arm, drastically increasing its size, then unleashes it for a devastating blow. This was first used to defeat Doflamingo, and was powerful enough to break through the Warlord's Kumo no Sugaki and God Thread techniques simultaneously, while still maintaining enough momentum to send him flying into the city below with enough force to completely level the town and cause the ground to fold in on itself. Using this technique causes the Haki patterns on Luffy's back to expand and cover his entire back with thicker patterns. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum King Kong Gun.
 * While in Wano Country, Luffy learned to advance his Busoshoku Haki and performed a new version of this attack. Using this version, he does not inflate his arm and instead projects Haki out of his fist. This allows Luffy to hit objects with an equivalent amount of power without even making physical contact. A single attack was powerful enough to shatter a large block of steel.




 * Gomu Gomu no Kong Organ (ゴムゴムの): A rapid-fire version of Kong Gun. After compressing both fists into his arms, Luffy moves his arms horizontally in a rapid fashion, with the afterimages giving the impression of two extra pairs of arms being materialized, before releasing a flurry of continuous heavy punches. This was first used against Cracker's Biscuit Soldiers, and each individual punch was powerful enough to smash through a biscuit shield. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Kong Organ Gun.


 * Gomu Gomu no Kong Rifle (ゴムゴムの): The Gear 4 version of Gomu Gomu no Rifle. After Luffy compresses his fist into his forearm, Luffy twists it before unleashing it on his opponent. It was first used against Kaidou. In the VIZ manga and Funimation adaptations, this is called Gum-Gum Kong Rifle.


 * Gomu Gomu no Kong Gatling (ゴムゴムの): The Gear 4 version of Gomu Gomu no Gatling. Unlike the Kong Organ, Luffy does not create horizontal after-images of his arm. It was first used against Kaidou. In the VIZ manga and Funimation adaptations, this is called Gum-Gum Kong Gatling.


 * Gomu Gomu no Over Kong Gun (ゴムゴムの): Similar to the King Kong Gun, Luffy compresses his enlarged fist and unleashes it for a powerful blow. However, it is now imbued with Haoshoku Haki, drastically increasing the attack's power. It was first used against Kaidou, forcing the Emperor's mouth closed, causing his fiery breath to combust in his maw. In the VIZ manga and Funimation subs, this is called Gum-Gum Over-Kong Gun. The attack's name also likely plays on the term "overlord" via the monkey king motif, given that 2 of the kanji in the attack's name can form the common word for it (覇 and 王).

Non-Canon

 * Gomu Gomu no Leo Rex Bazooka (ゴムゴムの・バズーカ): The Gear 4 version of Gomu Gomu no Gigant Bazooka and Gomu Gomu no Grizzly Magnum. Before Luffy compresses his fists as he does for Leo Bazooka, he blows a large amount of air into his arms, drastically increasing their size, then unleashes it for a devastating blow, similar to King Kong Gun. This was first used to defeat Gild Tesoro. Rex is Latin for "king" or "ruler". In the Funimation adaptations, this is called Gum-Gum Leo Rex Bazooka.


 * Gomu Gomu no King King King Kong Gun (ゴムゴムの): A more powerful version of Gomu Gomu no King Kong Gun. After Luffy compresses his fist, he blows a large amount of air into his arm to drastically increase its size—making it even larger than the King Kong Gun—before unleashing his fist for a devastating blow. This was first used against Douglas Bullet. In the Funimation adaptations, this is called Gum-Gum King King King Kong Gun.


 * Gomu Gomu no King Kong Gatling (ゴムゴムの): A more powerful version of Gomu Gomu no Kong Organ and rapid-fire version of King Kong Gun. After compressing both fists and blows a large amount of air into his arms, Luffy moves his arms horizontally in a rapid fashion, with the afterimages giving the impression of two extra pairs of arms being materialized, before releasing a flurry of continuous heavy punches. This was first used to defeat Douglas Bullet. In the Funimation adaptations, this is called Gum-Gum King Kong Gatling.

Snakeman

 * Gomu Gomu no Jet Culverin (ゴムゴムの): A faster version of Boundman's Gomu Gomu no Culverin. Luffy compresses his fist and releases it, using Python to change the direction of his punch according to the actions of his opponent. His arm speeds up the more it stretches, and the force of the punch was strong enough to smash Charlotte Katakuri into a wall. It was first used against Katakuri. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Jet Culverin.




 * Gomu Gomu no Black Mamba (ゴムゴムの): Luffy arches backward and compresses both arms before unleashing a rapid-fire strike, moving his arms up and down to give the impression of multiple arms moving to attack. As with Jet Culverin, Black Mamba involves Luffy changing the direction of his attacks, but their rapid movements make Black Mamba harder to avoid. Unlike its counterpart in the Boundman form, the Kong Organ, Luffy only manifests two pairs of afterimages instead of three pairs, indicating a lower frequency of attacks. However, he makes up for it with the semi-homing abilities of the Python, though it is not as accurate as the Culverins. It was first used against Katakuri, though he managed to dodge or block the majority of the attacks. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Black Mamba.


 * Gomu Gomu no King Cobra (ゴムゴムの): Luffy compresses his arm, and as he releases it, instead of directly targeting his opponent, he first has his arms loop around himself once before sending his enlarged fist flying at his opponent. The wide loop gives his attack additional acceleration, thus increasing its momentum, while enlarging the fist adds more mass, the combination multiplying the force of this strike. It was first used to defeat Katakuri, successfully knocking him out in a single hit. In the anime before striking the opponent, steam in the form of two snakes, one pink one blue, swirl around Luffy's arm before merging into a golden king cobra which dissipates as Luffy's fist enlarges. In One Piece: Stampede the technique is slightly different with several zigzags added to it for further acceleration. The kanji which reads as "ōja" mainly refers to a boa in Japanese. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum King Cobra.




 * Gomu Gomu no Hydra (ゴムゴムの): An attack consisting of rapid-fire punches similar to Black Mamba, but Luffy's arms are infused with Haoshoku Haki to increase their power. Unlike the Black Mamba, which uses the semi-homing abilities of Python and the rapid high frequency attacks to limit a more agile opponents' movements in a single direction, the Hydra uses advanced Kenbunshoku Haki to lock onto an opponent and barrage them with unpredictable, omni-directional attacks, which are nearly impossible to fully defend against or avoid, without using advanced Kenbunshoku Haki in return. The attacks are shown to come not only from the front, but from the sides and behind as well, effectively barraging the opponent in a 360° assault. It was first used against Kaidou, and was able to injure the Emperor and make him bleed. The Hydra was a multiheaded serpentine monster from Greek mythology and was said to grow additional heads for everyone it lost. Likewise, the kanji for it, "kuzuja", is also likely a play on the Japanese term "kuzuryū" (九頭龍/九頭竜; nine-head dragon). In the VIZ manga and the Funimation subs, this is called Gum-Gum Hydra.

Non-Canon

 * Gomu Gomu no Naga Urban (ゴムゴムの): Exclusive to the game Pirate Warriors 4. Luffy forms both of his arms into a drill-like weapon, which produces a steamy image of a serpent when he prepares to attack. When charged at full power, Luffy performs a twin drill punch by firing both of his arms at high speed. This attack is twice stronger than King Kong Gun and King Cobra, making it powerful enough to penetrate through Kaidou's Bolo Breath with an equal force. The Nāga are half-human half-serpent beings in several Asian religions, and in Japanese sometimes uses the kanji 竜 to describe them (differing from the kanji 龍 instead used to describe Eastern Asian dragons). "Urban" is possibly a reference to the Basilic cannon, which is known in Japanese as the "Urban Cannon" (ウルバン砲), after the canon engineer Urban. The localization of the game changes the name to Gum-Gum Dragon Culverin, despite the fact that it does not resemble the other Culverin attacks.

Tankman: Manpuku Version


The first time this form was shown, Luffy had consumed copious amounts of biscuits and consequently grew to an enormous and bloated size, and he referred to this version of Tankman as Manpuku Version (バージョン). In this form, Luffy resembles a sumo wrestler with exaggerated proportions: while his arms and legs are slightly increased in size with muscle, his Busoshoku Haki-coated torso becomes far larger and completely round, though still muscular near the top. The flame-shaped Haki also covers the bottom of his torso in addition to his shoulders and arms. This form also bears resemblance to Luffy's appearance when he combined the strength of both his Gear 2 and Gear 3 into one while fighting Gecko Moria (Gigant Jet Shell).

Tankman gives Luffy tremendous Haki-based durability: while Cracker nearly cut through Luffy's Haki-coated Boundman arm with his sword, he was unable to pierce Luffy's stomach with the same sword while the latter was in Tankman form. Furthermore, this form further increases the power and magnitude of Boundman's compression-based offense by using Luffy's enormous stomach as a launcher, which allowed him to defeat Cracker by blasting him through his incredibly hard Biscuit Soldiers with Gomu Gomu no Cannonball.

The known Devil Fruit attacks that Luffy uses in this form are as follows:




 * Gomu Gomu no Cannonball (ゴムゴムのキャノンボール): The Gear 4 version of Gomu Gomu no Fusen and Gomu Gomu no Orei Fire. After trapping an enemy within him by compressing and withdrawing his stomach, Luffy launches them forward and into the air with incredible power and speed. This technique was first used to defeat Charlotte Cracker, who, when launched, plowed through several of his immensely hard Biscuit Soldiers and was sent flying far away. In the VIZ manga and the Funimation adaptations, this is called Gum-Gum Cannonball.

Trivia

 * In Volume 79's SBS, it is revealed that Gear 4 is based on the fierce guardian entity called the Nio Statue, which fits well with Luffy's will to protect his crewmates with his life and beyond.
 * The steam generated seems to be semi-gaseous, resembling the ribbon of the Nio statue (inspiration of Gear 4). This behavior of the steam is included later in the manga.
 * This was further elaborated in Chapter 990, where even Hyogoro compared Luffy to a Guardian Deity otherwise known as a Myouou in Gear 4.
 * The anime added some details during the Boundman transformation which were absent in the manga:
 * In the anime this is the first of Luffy's forms to have a soundtrack made up especially for it. The theme is called "Gear 4" and is part of the One Piece New World Original Soundtrack.
 * Luffy´s Devil Fruit, the Gomu Gomu no Mi, can be seen in the background while his torso is being coated with Busoshoku Haki.
 * Similarly, to Gear 2, Gear 4 allows Luffy to use techniques that appear to be his own re-imagining of Rokushiki abilities, his pseudo-flight bears a striking resemblance to Geppo, and Leo Bazooka is seemingly reminiscent of Rokuogan (with Gear 2 allowing Luffy to move at a speed similar to Soru).
 * The concept of retracting limbs inward was first utilized by Kaku's Kirin Hodai.
 * Luffy being covered by Haki in almost all his body, except for the abdomen, chest and head and the Haki shadow trim around his eyes are similar to Dragon Ball series, Super Saiyan 4 (instead of Haki, the Super Saiyan 4 is covered by hair). In addition, both forms are named after the number 4.
 * Snakeman's Haki is colored in the manga the same way Boundman was colored in the anime. It is unknown if it was going to be its original color or if Oda changed it once he saw how Boundman was adapted to anime.
 * Boundman is featured as Luffy's Ultimate move in Jump Force.

Forms Specific

 * Boundman resembles Luffy's appearance when he became Nightmare Luffy (except for the hardened Busoshoku Haki 'tattoos', the enlarged biceps, and a more rounded torso) due to Luffy's increased muscle size, haircut's shape, and sinister expression.
 * The position of Luffy's hands to perform Leo Bazooka highly resembles the Kamehameha stance from the Dragon Ball series, a manga that highly inspired Oda.
 * Excluding Culverin and its variations, all Boundman's techniques are named after mammals, whereas Snakeman's techniques are named after snakes.
 * Incidentally, the word "culverin" is derived from couleuvre, the French word for grass snake.
 * It would seem that each form of Gear 4 has variations that Luffy has to pick carefully in a fight against certain foes:
 * Boundman seems to be balanced in all three categories, that this would often be the form Luffy turns into when Gear 4 is activated.
 * Snakeman is faster and lends itself better to "snaking" attacks, but sacrifices defense.
 * Tankman has the highest durability (making it ideal for defense) but has slower speed.
 * In the manga coloring, the steam emitting from Boundman and Snakeman are differently shaded, with Boundman having a purple hue on the cover of Volume 79, and Snakeman having an orange hue on the cover of Volume 89.
 * Boundman's steam was colored red and orange on the cover of Chapter 821.
 * When Luffy transforms into his Snakeman form in the anime, the steam takes the shape of several serpents, encasing him before the transformation is complete.