@Kishido:
Well maybe I'm too dumb or not insterested enough in this silly mode. But I still do not get the difference between gear 4 and 3…
Further more what would be gear 4 alike if CoA wouldn't be used? What exactly would be the difference between blowing up the bones or muscles?
The other way around what would be gear 3 alike if Luffy would use CoA, before blowing up his bones?
PS
It looks like shit. it's just silly that his "tatoos" a prefectly symmetric and looking like Luffy has been in a studio. Even more the eyes... Why the fuck are the eyes black as CoA hardening?
I think that gear 3rd is like the difference between swinging a fist and swinging a hammer. Your arm still has the same amount of power, but you're using a much heavier and sturdier tool. Because it's heavier your swing is slower.
Gear 4th is about increasing a different parameter: the amount of power in your arm. Imagine being all slim, but you're able to temporarily increase your muscle volume and instantly make yourself look like a body builder.
About the difference in usage, it seems like gear 4th is more for short-range attacks. We haven't really seen enough of it, but I'd like to think that gear 3rd still gives him the most attack power, while 4th balanced between 3rd and second (Doffy taunted him that 2nd lacks strength and that 3rd lacks speed), but being able to blow Doffy to the other side of the country, while we haven't seen anything of the sort from 3rd, makes it debatable.
If 4th is stronger, then the only good thing left about 3rd is that Luffy mastered it while 4th still has serious side-effects. I really hope it's not the case.
Other than that, Luffy was able to use king kong gun because 3rd and 4th are independent from each other, so there's that.
I don't know why Luffy needs CoA before he blows his muscles.. maybe it's hard for him to maintain the form otherwise? He deflated the moment he ran out of haki. Lucci was nicer than Doffy and bothered explaining how 2nd works.
I think that the black eyes are because his eye-sockets stretch when he transforms.