G4 didn't inflict any serious damage to Kaido unlike Doflamingo and it's obvious that Kaido is far from using his full power yet. Just the fact that Kaido was able of one-shotting G4 during their first fight, didn't even used his human or hybrid form against G4 Luffy clearly say that Kaido is far above G4 even with Luffy having unlocked advanced Armament.
Kaido one shotted Oden, that doesn't mean he was far above, it meant it was plot relevant, similarly Luffy was plot nerfed due to be begining of the arc (Croc, Lucci, Caesar Clown, Monet, all people not an untouchable distance above Luffy and sometimes weaker who gave him begining of the arc Ls), nerfed due to the fact he could not bypass scale defense and enraged and confused so FS didn't work.
Luffy isn't using his full trump cards either. Kaido got ragdolled and had to wait for G4 to run out, those are the facts.