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. The form is a testament to Luffy's natural combat ability to assimilate and adapt techniques, movements and attacks from his opponents. Not only does it utilize his own variation of Soru and Geppou, but the attacks in this form are directly inspired by fighting and befriending the strongest animals on Rusukaina, a lion, gorilla and snake.

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. 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 both Gear Second and Gear Third were ineffective in the fight. Luffy also shows a much more versatile side of the Gomu Gomu no Mi's powers by using his elasticity to achieve a form of pseudo-flight, changing the direction of his attacks mid-strike to hit a moving opponent, and strengthening his attacks in a manner similar to the Bane Bane no Mi by compressing his arms and legs like telescopic springs.

According to Doflamingo, this form is the fusion of Busoshoku Haki and Rubber. 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. Luffy has also shown to be able to quickly coat areas unprotected by Busoshoku Haki even when blind-sided, giving him a powerful defense.

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, 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 and as stated by Trafalgar Law, it overuses Haki.

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. 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）): 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, by bouncing it of the air itself using the same principle as his version of Geppou. This allows him to track his opponent after they were to dodge and still land a hit 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. This class of cannon was available in Water 7, and Usopp talks to the shiprights about the Demi-culverins, and they can be seen in the background.
 * 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.
 * Gomu Gomu no Leo Bazooka (ゴムゴムの獅子(レオ)・バズーカ): After retracting both of his fists into his arms, Luffy "fires" them at his opponent. This was first used against Doflamingo.

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.