Since the "reveal" in 783, some people have been sharing ideas about what gear 4th could be. That info is scattered across the chapter discussion, so I thought I'd start a more focused discussion here.
We've had gear 4th discussions before, but 783 gives us a new context to speculate from. Here are some things that we know about the situation that MAY be relevant to gear 4th:
(1) Gear 2nd is too weak ("It's like you're trying to tickle me!" -Doflamingo)
(2) Gear 3rd is too slow; Doflamingo can easily dodge and counterattack.
(3) Gear 2nd works on Luffy's blood
(4) Gear 3rd works on Luffy's bones
From these bits of information, it has been speculated that:
(1) Gear 4th has to be strong and fast in order to be effective against Doflamingo
(2) Gear 4th may work on Luffy's muscles
Either, neither, or both of those might be the case.
Concerning (1), the extent to which Luffy becomes fast and strong is up for debate - will it be as strong as 3 and fast as 2? Stronger than 3? Faster than 2? Or will it be stronger than 2 and weaker than 3, but faster than 3 and slower than 2 - a kind of moderate position between the gears?
Personally, I think that would be the most interesting option, as it wouldn't let him automatically stomp Doflamingo.
I've seen a lot of people talk about "compression" of Luffy's rubber body, or perhaps gear 4th making Luffy "unstable," and I even saw a fantastic four reference. Would anyone care to explain how that might work?
Besides the things suggested here, what do you think gear 4th will be, and how do you think it will work?