Really would like for Oda to use some BS excuse of "with CoC coating, my haki doesn't leak" and Luffy can maintain Gear 4 for longs periods of time, then he starts changing forms like Chopper Points.
Gear 5th would be some ultimate form that has all the maximum atributes from the different Gear 4th Forms, like Boundman strenght, Tankman defense, Snakeman speed, etc.