What is your opinion about Heroes IV?

Horn, no offense, but the second part looks like blatant advertising for Warlords of the West and Titans.
For example, I don't like either of them. I prefer the classic Heroes and find them more appealing. And, in my opinion, they have better balance than Warlords of the West and Titans. IMHO.

Swamp + Dungeon + Inferno = CHAOS? Why didn't they mess with Order or Humans instead?

It's very simple. Here's a brief historical overview.
Humans - they are humans everywhere. They could only be what they are.
Order - completely transitioned from the Tower. If you played the campaigns, you saw Gavin Magnus and Solmyr in action – the main characters from the Tower in the third game. So, they simply preserved those who were loyal to them.
Chaos. And they all perished.
Tatatia became slaves to Erathia.
The K'hara were destroyed during the time after the War of the Necromancers.
Dungeon - lost the war against Erathia.
As such, these factions no longer exist. They had their time. Something new had to be born. And Chaos became that.

Ivor - monk, there - priest

Specialization in elves. Both in the third and fourth games, he's a ranger.

In the third game, is the minotaur a naked bull-man like in the fourth?

We also have the Might and Magic series. In the dungeons of Nighon, there were indeed such naked bull-men, but I wouldn't call them weak monsters. They are the foundation of the army, both in the third and fourth games.