This occurs during the battle against Donquixote Doflamingo in the Dressrosa arc.

Luffy first uses in Episode 726 , titled "Gear 4! The Phenomenal Boundman!" . This momentous transformation occurred during his grueling battle against Donquixote Doflamingo in the Dressrosa Arc.

Luffy unveiled this "Full Stomach" version during his fight against of the Big Mom Pirates. When Does Luffy Get Around to Gear 4 in 'One Piece'?

Gear 4 is not just one form; Luffy adapts it into three distinct versions based on the combat situation. Gear 4 Form Key Feature Doflamingo Balanced power, speed, and flight Tankman Charlotte Cracker Extreme defense and counter-force Snakeman Charlotte Katakuri Unmatched speed and unpredictable angles Breakdown of the Different Forms 1. Boundman (Bounceman) - First Appearance: Episode 726

Luffy uses for the first time in Episode 726 (titled "Gear Fourth! The Bounce-Man").

In this episode, Luffy activates Gear 4 to fight against Doflamingo, also known as the "String Human-Devil Fruit" user, during their intense battle on the sky island of Skypiea's successor, the sky island Weatheria's branch, or more specifically, in the New World, at the country of Dressrosa.

After being pushed to his limit by Doflamingo's overwhelming power (including the deadly "Awakening" of his String-String Fruit), Luffy is thrown from the royal plateau. As he falls, he bites into his arm and begins to inflate his muscles not by blowing air into his bones (Gear 3), but by forcing air into his muscle tissue using Haki. The result is a hulking, bouncing form he calls "Bounce-Man."