Fourth Gear is a technique first seen in Luffy's battle against Donquixote Doflamingo. It was developed by Luffy during his training on Rusukaina.
Luffy coats his arm in Armament Haki before biting into his forearm. Similar to how he activates Third Gear, 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.
- Muscle Balloon: By coating his arm in Armament 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 Armament Haki enhanced kick from Donquixote Doflamingo, one of the Seven Warlords of the Sea, bounced back harmlessly, due to his body retaining its rubber properties despite being coated in Haki.
- Gum-Gum Kong Gun:
- n this form, Luffy's Armament Haki takes the shape of tribal flames on his body. This is the only example of Armament Haki forming a recognizable pattern.