For the last time, Gear 4: Boundman, means there were other forms.
Just like Gear 2 is the speed form, Gear 3 is the power form, and Fuusen is the defensive form, each of those has an equivalent in Gear 4:
Fuusen = Tankman
Gear 2 = Snakeman
Gear 3 = Boundman
Tankman is the Haki'ed up version of this
That's why I think he'll used it to block Kaido's club at some point in the arc, when he's stuck in a position.