In this form, Luffy's arms inflate marginally when a lot of the air seems to fill his torso. Luffy's durability rises sharply in this way, as he was capable of tanking and deflecting a piercing strike from Charlotte Cracker, While Formerly Cracker was able to Reduce deep into Luffy's arm https://pcnok85306.amoblog.com/the-smart-trick-of-when-does-luffy-use-gear-4-that-nobody-is-discussing-38414247