Kaido admitted that Red rock hurt him and he started wondering luffy's potential ceiling luffy thinks he will potentially achieve.. thats significant.. gear 4th didn't induce such a reaction even if you assume luffy used Acoa while using gear 4th..but it wasn't nearly as affective as red rock
huge reach.
kaido's reaction was to someone he deemed not worthy; growing in power (by learning CoA) and then hitting him with an attack that can atleast hurt him. it was in no way related to fire. if fire was the key aspect to hurting kaido, why didnt oden ever use any fire based attacks against him?
dragon kaido was pummelled to the ground with gear 4 + adv CoA. this was how dragon kaido was post that barrage.
no wonder he went hybrid soon after.
and mind you after Kaido turned hybrid luffy claims that his attacks aren't working meaning Acoa attacks aren't doing crap and then suddenly after hakai luffy charges and pulls a red hawk and kaido dodges it.. if it was "slightly hurting" him he wouldn't dodge it like he did against gear 4th Acoa and and Kid's robo Gipson thingi which are large scale and formidable.
he started dodging or actively trying to dodge luffy's attacks post the g4 barrage. since the advanced armament can hurt him even if it's slightly.
here's luffy himself telling that advanced armament was too shallow.
from this you can deduce that the fire element luffy generates (whether spirit or science or both) while imbuing haki in it affects kaido significantly more than common Acoa punches or even gear 4th attack that are just blunt and get absorbs.
the attacks that did the most damage to kaido on panel ever - oden's togen totsuka, luffy's adv CoC punches and zoro's ashura - all had nothing to do with fire. oden's togen totsuka had adv CoC though.
it's absolutely headcanon if you argue that advanced CoA + fire can do more damage than G4 + adv CoA to kaido. that is factually false since G4 pummelled kaido to the ground in his most tankiest state - the dragon form.