GEAR 4: BOUNDMAN
Luffy’s first Gear Fourth 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 bounces on the spot.
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.
GEAR 4: TANKMAN
During his battle with Charlotte Cracker, Luffy revealed another form of Gear Fourth known as Tankman.
Due to the increase in Luffy’s metabolism Gear Fourth 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.
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.
GEAR 4: SNAKEMAN
During his battle with Charlotte Katakuri, Luffy revealed the third form of Gear Fourth known as Snakeman. Unlike his other two Gear Fourth forms, Luffy does not significantly expand, with his forearms and legs 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. The steam produced from his body 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 to not only press an attack more fiercely on an opponent capable of dodging, but increase the attack’s speed and power until he finally lands the hit, and 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.
However, because of his body’s lack of inflation, Luffy’s defense is considerably worse compared to his Boundman and Tankman forms.