Gomu Gomu no Mi/Gear 4 Techniques

Gear Fourth: Boundman is a technique 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 Third, 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.

Luffy's proportions become warped and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. The coating across his torso resemble the wispy ends made from flames, giving it the appearance of tattoos, and he also gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall. Like when using Gear Second, Luffy is continuously emitting steam. As a side-effect, he is incapable of standing still on the ground, and instead constantly bounces on the spot. This form was created in order to combat the many powerful and large animals on Rusukaina. This form (except for the hardened Busoshoku Haki 'tattoos', the enlarge biceps and a more rounded torso) resembles his appearance when he became Nightmare Luffy due to Luffy's increased muscle size, haircut's shape and sinister expression.

While Luffy is using Gear Fourth, his physical strength and speed are increased to the point where he is able to overwhelm a member of the Shichibukai, Donquixote Doflamingo, whereas before, Gear Second's speed and Gear Third's power were ineffective in the fight. Unlike the previous forms, Gear Fourth focuses on compression more than stretching. While the compression grants Luffy's moves overwhelming speed and power, the range of his attacks is severely decreased, though he can 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 shows a much more versatile side of the Gomu Gomu no Mi's powers by changing the direction of his attacks mid-strike to hit a moving opponent without losing momentum.

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.

This form, much like Gear Second, 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 bounces 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.

However, there appears to be a limit to how long Luffy can keep Gear Fourth active, as noted by both Luffy himself and Doflamingo. Once this limit is reached, Gear Fourth is automatically deactivated, 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. Even afterwards, Gear Fourth is extremely taxing on Luffy's body and stamina; after regaining his Haki from using Gear Fourth once, he barely had enough strength left to fight back, and using it briefly for a second time shortly after rendered him unconscious.

Techniques Utilized

 * Kinniku 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 kick from Donquixote Doflamingo, a Shichibukai, bounced back harmlessly, due to his body retaining its rubber properties despite being coated in Haki.
 * Gomu Gomu no Kong Gun (ゴムゴムの猿王銃（コングガン）): After compressing his fist into his enlarged forearm (and making it look like the barrel of an actual cannon), Luffy unleashes a devastating short-range punch with massive power. This technique had enough strength to break through Doflamingo's own Haki-enhanced defense and blow the Shichibukai all the way from Dressrosa's palace to the middle of the city.
 * Gomu Gomu no Rhino Schneider (ゴムゴムの犀榴弾砲（リノシュナイダー）): After compressing both legs into his body, Luffy delivers a double flying kick, similar to Gomu Gomu no Yari. 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. A howitzer is a type of short-barreled artillery cannon, and "Schneider" refers to the Canon de 155 C modèle 1917 Schneider, a howitzer used by several countries during World War I.
 * Gomu Gomu no Culverin (ゴムゴムの大蛇砲（カルヴァリン）): Similar to Afro Luffy's Gomu Hook, but improved. Luffy delivers a punch like a regular Gomu Gomu no Pistol, but if it is dodged, he can redirect his punch an unspecified number of times, without losing momentum, to still land with tremendous force. In addition, this punch does not need to retract back to his body right after being "fired" as the regular Gomu Gomu no Pistol does. A culverin is a relatively simple firearm, ancestor of the musket and cannon.
 * Gomu Gomu no Double Culverin (ゴムゴムのダブル大蛇砲（カルヴァリン）): Luffy pulls back his arms before punching his opponent with them. This was first used against Doflamingo, but was countered by his Offwhite.
 * Luffy used a Gear Fourth version of Gomu Gomu no Kane, but hasn't named it yet. Luffy retracts his head into his torso and then delivers a powerful headbutt to his target. It was first used to break Doflamingo's Billow White so he can strike then with Leo Bazooka.
 * Gomu Gomu no Leo Bazooka (ゴムゴムの獅子（レオ）・バズーカ): The Gear Fourth version of Gomu Gomu no Bazooka. After retracting both of his fists into his arms, Luffy "fires" them at his opponent. This was first used against Doflamingo.
 * Gomu Gomu no King Kong Gun (ゴムゴムの大猿王銃（キングコングガン）): Luffy blows a large amount of air into his arm, drastically increasing its size, before compressing his fist and unleashing a devastating blow. This was first used to defeat Doflamingo, and was powerful enough to break through the Shichibukai's Spider's Web and God Thread techniques simultaneously, as well as send him flying into the city below with enough force to completely level the town and cause the ground to be torn apart.

Trivia

 * In this form, Luffy's Busoshoku Haki takes the shape of tribal flames on his body. This is the first example of Busoshoku Haki forming a recognizable pattern.
 * 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.
 * The cover page of chapter 784 portrays the Straw Hat pirates having a picnic with a group of zombies. This was likely added by Oda as a nod to Thriller Bark, where Luffy activates his "Nightmare" form, which Gear 4th bears a striking resemblance to.
 * Similarly to Gear 2nd, Gear 4th allows Luffy to use techniques that appear to be his own reimagining of Rokushiki abilities, his pseudo-flight bears a striking resemblance to Geppo, and Leo Bazooka is seemingly reminiscent of Rokuogan (with Gear 2nd allowing Luffy to move at a speed similar to Soru)