What would you change in Heroes of Might and Magic IV?

No, I don't think so. I think the graphics of the creatures are much better. At least the vampires in the third game are simply unbearable to look at, while in the fourth game they are amusing.

I don't honestly remember what the vampires look like in the fourth game, but I definitely remember what the dragons look like (which should be the most beautiful creatures) - they are fat cows with large behinds and wings (exaggerating). It's disgusting - they look better in the third game.
And in the third game, the vampires are normal, although I didn't pay much attention, but the overall impression is positive.