Not in the same episode (that’s 727–728), but 726 ends with Luffy compressing his arm and declaring “Gomu Gomu no…”, leaving you on the edge of your seat.
This form focuses entirely on defense and counter-force. When Cracker attempts to stab Luffy’s distended stomach, the incredible tension "swallows" the blade and launches Cracker across the island with the move Gomu Gomu no Cannonball . The Speed Specialist: Gear 4 "Snakeman" (Episode 870) When Does Luffy Get Around to Gear 4 in 'One Piece'?
Luffy bites into his forearm and pumps air directly into his muscles (unlike Gear 3, which pumps air into bones).
He coats his body in a dense layer of Armament Haki to maintain the tension and shape of the air within his muscles.
The Gear 4 transformation has become a fan favorite in the One Piece series, showcasing Luffy's incredible abilities and his growth as a powerful pirate.
Some of the key features of Luffy's Gear 4 transformation include: