3 has everything good that was in 2, and some more benefits.
4 I did not like, not at all. Played the demo and was not enthused. I keep playing 3 over again, go through all the campaigns maybe once every two years.
Dungeon is my fave. Several good flyers, a couple great archers (no melee pentalty, special effects) and one solid walker. Nice town bonuses too, like mana doubling, free 1000xp (great for new heroes just starting out) and artifact merchant, which is good for a quick boost to starting heroes and also selling useless junk. The mage guild goes up to 5 which is nice cuz not all of them do.
Rampart is nice too cuz of the several solid flyers, better than average walkers, an archer that upgrades to double attacks, also resource bonuses like the pond and gold bank.
Wizard is nice because the 1st and 7th tier gain ranged on upgrade, and they have extra spells in the mage guild.
Barbarian and Demon arent great. Too many weak units holding everyone else back.
Swamp is fun cuz they have some annoyingly effective units like serpents with the poison, basilisks with stone, and gorgons with death attack, also hydra with circular damage. But I was never too effective with them. Ditto Undead.