Problems with Heroes of Might and Magic IV.

First of all, there aren't any clearly outstanding strong castles and creatures (unlike in Heroes 5, where individual Druids or Rangers can wipe out an entire army). Secondly, there aren't any magic spells that create imbalance, like Flight or Teleportation – the latter is the most annoying of the three. Furthermore, stronger units move slower than other units of the same level (like the Ballista), which makes stronger armies usually slower, and armies consisting only of mobile units more agile, giving them a certain advantage. I can't think of any more examples right now, but there are others...

Well, Heroes 3 doesn't have that either, and you're exaggerating about Heroes 5. In that case, you need to take appropriate units from other castles. The Flight and Teleportation spells are very limited and can be quite difficult to obtain in a normal scenario; they are mainly used in campaigns. But the cost of such magic is also high, which is why it's high-level magic. In Heroes 3, units also move according to logic. For example, Phoenixes, Fairies, and Archangels are the fastest, while Golems, Zombies, and Hydras are slower (for obvious reasons). In Heroes 4, the Poison spell is annoying because it affects everyone, especially hero-mages.
Furthermore, I can't call Heroes 4 more realistic, either in terms of gameplay (when one hero can defeat a huge army) or appearance (sometimes a medium-sized tree is larger than a city or an army).
I'm defending Heroes 3, but I also liked Heroes 4 in its own way. It definitely has great potential; perhaps if it had been released under a different name, like "From the World of Fantasy" or "Slaughter in the Kingdom," it might have become Game of the Year. BUT this is Heroes, so it's not surprising that it's being compared to its predecessors. However, the presentation of the CAMPAIGNS is very good and is a level above the previous installments, and the main strength of Heroes 4, for me, lies in the presentation of these campaigns, in these quests built on a clear editor, with which you can create a completely unpredictable scenario using dialogues and sequential events. Honestly, when I was playing, I read all the dialogues and played to follow the development of events and then to find out what would happen next. The game took a huge step towards RPG, but at the same time, it wanted to remain Heroes, namely a turn-based strategy game. I love RPGs, but there's some ambiguity here, which is a bad thing.
Again, IMHO.
P.S. Commanders and generals shouldn't be in the thick of the battle; they've been given too much freedom.
P.P.S. And now, what was the purpose of this post? It's silly to reject Heroes 4 because it's a good game, but it's also silly to say that it's the best. It's just a good game.