Oh man, Gear Fourth.
Get yer guesses in what Gear Fourth will be like.
Gear Second - enhanced strength and mobility through accelerated blood circulation.
Gear Third - inflated bones for hard, massive attacks
Since those two are essentially speed and power, perhaps 4th will be technique. Use the Gomu Gomu no Mi in inventive ways. If he wanted to, Luffy could stretch the lens of his eyes and his retinas to gain better-than-telescopes-or-microscopes super-sight. Maybe he can stretch his brain and gain an analytic edge. We know Luffy's not much of a thinker, so it would be more instinctual, like time appears to slow for him. Or maybe Luffy uses his flexibility to redirect force, not unlike Spike from Cowboy Bebop's use of Jeet Kune Do (
"Be like water").
Beats some of the stuff I've seen on the subreddit. One suggestion had him using "vulcanization." Why would he develop a hard rubber technique if he already has haki (color of armaments)? What do you think Gear Fourth could be like?