I think that Luffy will have four Gear 4th forms and he will use his final unrevealed form in the fight against Kaido.
I believe the last form we are missing is Gear 4: Lion Man.
Here are the reasons as to why I think this is Luffy’s last Gear 4th form and what kind of improvements this form gives him. First, I believe Tank Man is his defence form and the one off in this naming scheme.
His other 3 forms are his attack forms. Luffy’s Gear 4 forms take inspiration from animals. Not just any animals, but animals that are referred to in one way or another as Kings. King Kong, King Cobra, and King of the Jungle. And what 3 animals were on Ruskaina that Luffy didn’t eat because he became friends with? That’s right! The gorilla , the lion, and the… crocodile… it’s a reptile, let’s just pretend it was a snake.
While fighting these animals, Luffy had to use Gear 4 to take them down. I believe he used Bound Man to defeat the gorilla, Lion Man to take down the lion, Snake Man to take down the crocodile/ snake, always adapting his fighting style to be better suited to taking down each different animal.
Luffy’s other Gear 4th forms could have also been foreshadowed in his fight with Doflamingo. In that fight, he used his Python technique, which later was shown to be more potent during his Snake Man transformation.
He also used a move called Gomu Gomu no Leo Bazooka.
See where this is going? Much like how Python was stronger during Snake Man, Leo Bazooka will be stronger when he uses Lion Man. It seems that Bound Man is his base Gear 4 form, a balance between strength and speed, Snake Man is his straight speed form, and Lion Man is pure strength form. It would also be poetic if Luffy takes down Kaido, The King of the Beasts, in the form of the actual King of the Animals.
*Theory by walidnusseiri