when does luffy use gear 4

Luffy first uses Gear 4 in episode 726 of the One Piece anime, during his battle against Donquixote Doflamingo. He needed Gear 4 because his previous Gears 2 and 3 failed to deal sufficient damage to Doflamingo. Gear 4 dramatically enhances Luffy's strength, speed, and defense by inflating his muscles and coating his arms and legs with Busoshoku Haki. This form allows him to defeat powerful opponents that surpass the capabilities of his earlier Gears. Gear 4 was developed by Luffy during his two-year training on Rusukaina Island with Silvers Rayleigh. It combines the rubbery powers of the Gomu Gomu no Mi with Busoshoku Haki, inflating Luffy's muscular structure, and granting him multiple distinct forms tailored for different combat situations such as Boundman, Tankman, and Snakeman. Even though Gear 5 has appeared later in the story, Gear 4 continues to be a vital technique, now even mastered to use in parts of his body selectively, improving stamina and Haki usage. Gear 4 has a time limit and drains a lot of Luffy's energy, rendering him exhausted and unable to use Haki for several minutes after use, but its effects have been lessened after further training. The usage of Gear 4 marks Luffy's upgrade to tackle increasingly powerful foes in the New World and beyond. In summary, Luffy uses Gear 4 first against Doflamingo to surpass previous limitations, and continues to utilize and master it for key battles throughout the series, especially when greater strength and speed are required.