Can anyone answer why G4 didn't hurt Kaido when G3 could do some recognizable damage?
Luffy didn't use barrier/penetration haki with G4?
Or any other reasons? can anyone answer this...
I think Oda did this for the Luffy - Kid - Law dynamic.
We could see it with the gag against Promotheus' attack, so i think the sequence that happened after that was just a follow up of that, with Oda taking advantage of this to showcase Kid and Law.
He didn't need for Luffy to eclipse Kid and Law's showings yet since he already made the build-up through the red rock that not only Luffy can damage Kaido, but could potentially be among the people who could outright match him
Luffy is reserving his all for later.