Rainer
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HoMM4: Dungeon & Dragons - an "unofficial addon," also known as a pirated version.
The AI in "Heroes 4" really isn't that great.
I recommend installing the "Equilibrium" mod (3.51, or 3.55. The latter is preferable. In it, the AI doesn't die on neutral creatures and behaves more actively).
PS: Maybe it will help :)