I read in a neighboring thread that playing Heroes IV mainly involves battles with 5-6 heroes at levels 12-15. I went through about twenty scenarios and didn't see any indication of such a progression. There are few chests, and neutral creatures don't give much experience. So, it's unclear how this is done.
I apologize if this has already been discussed; just point me to a link.
Yes, on standard maps in scenarios, you can't really level up many heroes, as they were originally created not for multiplayer (with a few exceptions). I understand that most online maps are created (or reworked from existing ones) by fans. I don't fully understand how this is done yet, as I have little multiplayer experience, but theoretically, this is the best approach.