I want to believe that there is G5, but the question is what is Luffy gonna use? Is G2, G3 and G4 the combination of G5?
Luffy got used to G2 and he still at high stamina level after using it. It's just that his body still not used to G4 that much like G2. So you better expect to see G4 getting used a lot in next arcs after Dressrosa. What would bothers me is to see Luffy continue using G4 even after opponents like Doffy. G4 here and G4 there.
Keep in mind. Luffy used G4 just for training with beasts at Amazon Lily. Creatures as powerful as Doffy. All next arcs are going to be villains at Doffy's caliber or a bit close. Until Luffy masters G4.