The final move of Gear Fourth: Model Snakeman will be for Luffy to unhinge his jaw and eat Katakuri whole.
In subsequent chapters, Luffy won't get his usual post-meal fat belly, but rather, a lump larger than his body which starts in his neck in the scene after he wins.
In the subsequent scene, it will be slightly smaller and in his shoulders.
In the scene after, smaller still and in his torso.
When they arrive at Wano, Luffy will then reveal the secret technique of snakeman: molting. He will shed his skin and the new Luffy will emerge: he looks just like current Luffy but a bit older, and Garp's height.