Predictability of the AI.

Why is it dumber in the 4th game??? What's the problem? Why couldn't they make it like in the 3rd game?

In the 3rd game, you have money, so you can buy heroes whenever you want until you reach the limit. The AI doesn't seem to have any money problems there. Therefore, it can always have a lot of heroes and attack in all directions. But I wouldn't say that its actions are particularly smart. In the 4th game, you can only buy ONE hero in a week in the fortress. Of course, the AI doesn't risk losing it, although it sometimes does lose heroes even against neutral creatures while trying to develop. Unlike us, the AI doesn't know how to retrieve its hero's corpse from under a monster to revive it. Therefore, it sits and waits a week to buy the next hero. Then it can only reclaim the corpse of its main hero. In short, making the AI smarter in the 4th game is a task for the map designers. For example, guarding the AI's spawn point with weaker neutral creatures, and guarding the human player's spawn point with stronger neutral creatures.
Conclusion: The AI is equally dumb in both the 3rd and 4th games. It's just that in the 3rd game, due to the economy and the continuous purchase of heroes in the fortress, the AI seems smarter. :)