Luffy officially triggers Gear 4 during his rooftop duel against Donquixote Doflamingo in Episode 726, titled "Fourth Gear! The Phenomenal Boundman!" Manga readers witnessed this turning point earlier in Chapter 784, published in April 2015. Up to this encounter, Luffy's conventional offensive lineup, Gear 2’s blood-pump speed and Gear 3’s inflated bone strikes, failed to breach Doflamingo's awakened string barriers or inflict permanent damage.
With the Birdcage shrinking over the civilian population of Dressrosa, Luffy bit into his forearm, coated his torso and limbs in jet-black Busoshoku Haki, and inflated his muscular structure. The result was Boundman (also translated as Bounceman). The form overwhelmed Doflamingo immediately. Luffy's speed exceeded Doflamingo's kinetic perception, and his Culverin punches redirected mid-flight through air compression. The sequence shifted the anime's visual style, framing Luffy as an imposing, bouncing guardian deity modeled after traditional Japanese Nio statues.
