Monkey D. Luffy has gone through a lot during One Piece's nearly 25-year serialization. He has faced countless enemies, met lots of companions, and won myriad battles. Naturally, he has become stronger as the story has progressed, and he manifested his experience through his gear power-ups. Gear Two and Gear Three only have one or two varieties, Luffy's Gear Four comes in various shapes and sizes. Here are all of his Gear Four versions.
Bounce-Man was the first Gear Four variation to be revealed. Luffy used it to defeat Doflamingo during their battle at Dressrosa. Activating Bounce-Man makes Luffy's body proportion warp in a weird way. He becomes inflated, similar to when he uses Gum-Gum Balloon. Luffy's arms and feet tough in contrast become buff, though his feet are noticeably smaller compared to the other parts of his body. They are also covered in Color of Arms Haki along with his arms.
Luffy created this version to counter the gigantic animals residing in Ruskaina. Bounce-Man offers formidable defense as it can bounce back attacks to a certain degree, while his Color of Arms Haki lets him stop cutting attacks to an extent. The balloon-like proportions allow him to "fly" by floating and repetitively kicking the air to move. Doing this enables Luffy to move at an extremely fast speed, though it does come with a drawback. Luffy can't maintain his balance due to Bounce-Man's odd proportions, resulting in the need to keep bouncing on the ground.
Unlike Bounce-Man, there is limited information available about Tankman. Luffy first used this during his fight against Charlotte Cracker, wherein he consumed a lavish amount of biscuits making him grow in size. However, what Luffy activated then wasn't the base form of Tankman but its Stuffed Version.
In this version, Luffy's bloated torso becomes completely round with Color of Arms Haki covering most of his bloated stomach. Luffy's limbs, although also enlarged, seem small in comparison. Tankman: Stuffed Version boasts great durability, as shown when Cracker failed to pierce through Luffy's stomach, while also amplifying Bounce-Man's compression-based attack using his humongous stomach as a boost.
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Yet another Gear Four version Luffy pulled out against one of Big Mom's Sweet Commanders is Snake-Man. In contrast to the two previous forms, Luffy does not exaggeratedly expand when using Snake-Man, though his torso and limbs still do become bigger. Similar to Bounce-Man Color of Arms Haki also only extends to his forearms and legs. The more prominent change in this form is Luffy's knuckles. Instead of clenching his hand into a fist, he forms a Leopard Fist by flexing the first joint of his hand.
Snake-Man seems to be an offense-focused form, allowing Luffy to strike agile opponents. By using Python in this version, Luffy can alter his attack's direction in unexpected ways. The longer Snake-Man's attack lasts, the faster it becomes. Luffy optimized this ability by employing it in tandem with Color of Observation Haki, giving it a homing element. Snake-Man, however, does not offer any boost in defense unlike the other Gear Four versions. While he doesn't become weaker, he simply retains his defensive ability prior to transforming.
Luffy's Gear Four is his most varied gear by far, and he still hasn't shown its full capacity. Tankman's base form, for one, is yet to be revealed. It's reasonable to assume that he has more Gear Four versions up his sleeve, given that One Piece has yet to reach its final chapter.