Right now, I'm only playing the fourth game. I've even put down the second part, which I used to hold in high regard.
In my opinion, the fourth game is the best in the series. I know I'll get flak for this, BUT... playing the 2.0 version from the Russian localization, I don't notice any major balance issues... Maybe I'm just not a "pro gamer," and I don't pay close attention to certain nuances, but playing via hotseat and against the computer, I don't have any complaints. Neither the balance of races, nor the balance of creatures, nor the balance of schools of magic raises any particular concerns.
And now, let's consider the best interface in the series, the most pleasing graphics, and excellent design (the third part doesn't even come close with its plastic dolls and puppet-like animation - that's just my harsh opinion), the best system of schools of magic and ranks/classes/skills, and finally, the magnificent music - the result is at least a simply excellent game.
There are just too many changes compared to the third game, and the hardcore fans weren't ready for it - many still aren't ready ))) and they keep bashing the fourth game.
But it doesn't deserve it...
Please remember that what I've written is just my opinion.
I've played all the games and quite a bit.