I agree that 4 got a bit of a raw deal. It just changed too much with the heroes on the battle field as a unit, and of course the AI had some difficulties, though the campaigns were fun enough for me.
It was different, some good some bad, but not really close enough to the spirit of the other HoMM titles and I think that's what worked most against it.
5 was great though. 3 was ok for me, but I felt they incorporated alot of time savers into 5 to make the game play more smoothly (less need for hero trains for one thing), and the tac battles were definitely more fun.
Well we'll see how 6 plays out, doubtful I'll be in on day one in any case, but I'll remain hopeful that they won't screw up too much (DRM non withstanding there)