G4 is a nerf at this point yea
Doesnt do better than base and puts Luffy on a timer
He's already on a timer according to Yamato, he's barely standing after eating some food, he's not capable of a protracted fight.
Kaido is keeping lots of cards close to his chest, he only started using FS after hydra overwhelmed him. This comes off as quite vague and hard to tell, but we are seeing Kaido doing crazy culverin style life return techniques as a result of Luffy pushing him.
G4 made Luffy completely dominate Kaido for several pages without him having a chance for response, that is a boost compared to before, but just like against Kat it didn't immediately win him the fight, Luffy was fighting Kat for an hour fairly evenly after learning FS before entering snakeman iirc- this pushed out Kat's donut trump card.
G4 looked way more dramatic of a boost against Doflamingo, than it did in subsequent arcs, but it is a big boost still.