Four is way too much. Kenzan shows that, Ishin shows that, 0 shows that, K1 shows that, and LJ shows that.
Every style has some singular gimmick to differentiate itself from the others, one of them is always overtuned to an insane degree that it invalidates all other styles (2kat, Pistol/Wild Dancer depending on if Ishin or Kiwami, Beast/Breaker, Dragon, and Boxer), and worst of all is that the styles never feel as finished as DoD, Akiyama's, Saejima's, Tanimura's, or Shinada's styles. They're always missing so many contextual Heat Actions either in the environment or through specific items, they're always feeling like they have half the moves of the "main" style, and even the "main" style feels like it should have way more moves that it would have had if not for having multiple styles.
8G and The Kaito Files even showed two is too much with Mad Dog being so barebones and lacking any reason to stay in it compared to Sea Dog which was clearly the main focus (Even if you lose grabs and environmental Heat Actions) while The Kaito Files gave you no real reason to ever use Tank over Bruiser with both styles in general being extremely barebones, weak, and feeling like you were in control of a random NPC that could use some moves from Brawler or Beast.
The style system, in my opinion, shouldn't exist. It takes away the focus they had going on with DoD for years to split into so many superfluous styles, and LJ was the only time it ever made sense with the passive buffs you got for doing different actions, but even that meant you stuck to one style like Tiger/Boxer 90% of the time the further into the game you got because there was no reason to ever use Snake or Crane since they both became so underpowered.