Campaigns let you down, but single map adventures are a lot of fun
When Heroes 3 came, you could choose to mess around with single-player maps. Beef up your towns and troops and put treasure out near your towns for easy loot. And then save the changes on the default map. Easy cheating - instead of using in-game cheats.
This you can of course do with Heroes 4 as well. But now you can beef up your heroes so that they maybe don´t even need any henchmen or monsters to travel with them. My fav is a downloadable version of Might and Magic VI - The Mandate of Heaven. Now you can just go through the map with an party that just consist of up to 8 heroes. Play just how you wanna play. This game has actually made "mild cheating" fun.
But the campains are NOT as good as in H3 - in all my time spent with this game I have only played through 2 (I think). They seem to lack something that makes you throughout enjoy playing them. I tend to just get bored.
The music is mostly great as in its predecessors (yes even the midis in H2 had it's great moments). Both when you explore maps and when on the town-screen.
Overall, not as great as H3 (MOSTLY because of the weak campaigns) - but still tons of fun to play and easy to get into. And if you get bored with the maps that comes with the game when you buy it - just get on the net and download some more. People are still making new ones even today. And then you can mess around with them in the mighty map-editor (or make your own maps of course).