My first game was the third installment.
And after that familiar third game, I looked at the fourth one :(
I didn't like it for these reasons:
1. Few races, and even the names are silly.
2. I didn't like that troops can move without a hero.
This means you can distribute ten imps into ten armies and have them collect all the easy loot, go to enemies and collect loot from them.
3. I really didn't like that there are very few creature levels (only 4, right?).
4. I didn't like the selection (of the last level).
5. I really didn't like the daily growth (it's just absurd).
You can buy troops every day and send them to the hero.
6. There are fewer artifacts, and there are no composite artifacts at all.
7. After the third game, mixing troops in cities seemed ridiculous.
8. The battle has an idiotic mechanic - a ranged unit cannot shoot at an enemy unit if another unit is standing in front of it.
9. I didn't like the graphics - it's just garbage, not pseudo-3D.
10. The buildings inside the cities don't look good at all. If in the third game, as the city was built, each building complemented the others, then in the fourth game, the building pictures are just randomly stuck together.
That's all my IMHO