User blog comment:Bluefox87/Regarding the aftereffects of gear 4th ch 785/@comment-5398287-20150513170717

Given how the consequences of using the previous two gears were effectively causing Luffy to experience the opposite of whatever they granted - decreased stamina and strength for Gear Second, which increased speed and power; decreased size and strength for Gear Third, which increased size and power - for however long, Gear Fourth most likely has a similar consequence. Since Luffy is increasing the size of his muscles, I'm betting that once Gear Fourth turns off, his musculature is gonna undergo extreme atrophy, to the point where he'll be effectively skin and bones, such that he won't even be able to move for a while - and before anyone asks about how this would be possible if it's only temporary, recall that Gear Third puts Luffy in a chibi form for a given period of time without explaining where all the excess mass goes, and also that we're reading One Piece.