Luffy's huffing too. Literally after the panel of Killer huffing we see Luffy huffing as Zoro shouts at him. Bearing in mind only one of the 5 rooftop fighters got a lovely 10 minute nap whilst being protected by Zoro.
The smoke is not from G4, it's from Luffy being thrown into the rocks. We know this because a) the smoke isn't coming from Luffy's mouth initially. We see him screaming in the very first panel of him and there's no smoke from his mouth.
B) the first line of smoke we see initially has the exact same art style as the second line of smoke, which occurred after Luffy attacked without using G4 and jumped up shouting "Gomu Gomu no..." Only to get knocked back into the wall.
When Luffy finishes a full round of G4, the smoke emanates from his entire body and the volume of smoke is infinitely larger than the small puff we saw here. It's possible he did enter G4 a 2nd time at some point as Kaido has been damaged and it's mainly Luffy and Zoro who are capable of that, however, in this particular scene he was fighting in G2/3.