Boundman (Tension Man)This is the default version of Gear 4th. It focuses on a balance of defense, flight, and explosive power. In this form, Luffy’s physique becomes top-heavy and incredibly springy. He cannot stand still and must constantly bounce. His signature move, the Gomu Gomu no Kong Gun, compresses his fist into his forearm like a spring before releasing it with devastating force.

When Luffy activates 4th Gear, his body undergoes a radical transformation. His muscles expand, and his body becomes covered in a network of inflamed, red veins. His limbs elongate, and his joints become incredibly flexible, allowing him to move with enhanced speed and agility. Additionally, Luffy's physical strength increases exponentially, making him capable of delivering devastating blows that can crush even the strongest opponents.