Predictability of the AI.

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

In my subjective opinion, it's simple. In the 4th game, there are many more opportunities for development and movement around the map (units move differently, they all have different speeds, and they all move differently on different types of terrain, etc.).
If you add a couple of difficulty levels for the AI and remove the first two, then only IronAxe will be able to play on the maximum difficulty.