I bought HOMM4 when it came out, but while I loved HOMM3 I can only say that I liked HOMM4.

I would not say that it is a bad game (I actually enjoyed it) but it does not quite live uo to its predecessor.

One of the main differences is that the heroes now actually fully participate in battles (i.e. they are units that can fight hand-to-hand and can actually get killed).
While this sounds like a logical development from HOMM3 it does not work out quite well - at least IMHO. The problem is that the heroes either seem to be overpowered or they can be so weak that they get killed to easily (which might lead to the instant end of a campaign (or better a reloading of the last saved game).

Also I think the siege battles are much less fun than in HOMM3.

On the positive side - the development of the heroes got much more detailed and interesting (more skills to choose from, higher classes to advance to) and all the monsters have some special abilities.

Overall a fun game, but I still find myself playing HOMM3 more often than HOMM4.

But as you will probably get it for a very low price now I would go for it as it still gives lots of fun as might be a got way to get yourself in the mood for the upcomung HOMM5