What is your opinion about Heroes IV?

I also don't keep more than two heroes in one army. You won't get enough experience for everyone. And no one forces you to take more heroes than you need. In general, having multiple heroes is a useful thing – you distribute artifacts among the heroes, and each hero of an advanced class in one army will give it its own bonus, etc.

This is just my opinion, but it's worth taking at least 4 heroes, and each should level up in their own area of skills (like 1 runner, 1 tactician, and a couple of mages). This is very convenient on online maps; by the endgame, the heroes are almost always at least level 15, and there are 5 of them, not 1. I used to play only with a single hero, and that was fine, but the first online match showed that 4-5 heroes level up at almost the same rate as 1. One level 18 hero against 5 level 14-15 heroes will lose in 100 percent of cases, even with an army 2-3 times larger. That's all.