DisciplesII -vs- HOMM4 is like comparing Q3(fairly simple gameplay) -vs- Q3 weapons factory(lots of strategy choices).
Disciples 2 is fairly simple. You travel-fight, travel-fight, with only a few tactical decisions to make.(i like the enemy AI better, they trash talk)
HOMM4 is much more complex, alot more choices to make, which hero skills to take, alot more spells, and which group of units to carry into battle. The downside is switching out units is almost too simple. In Disciples switching out units means you start with a new creature at lvl 1. (HOMM4 has sooooo many choices to make as you play, i restarted the first map (3) times after clearing it, to see how strong a different group of heros would be)
If you like a simple strategy battle game get Disciples2.
If you prefer a game with LOTS of choices, and strategy get HOMM4.
Personally i would play through Disciples2 and then buy HOMM4 afterwards.
(If you played HOMM4 first and Disciples2 second it may seem way too simple.)