if you like civ rev, it is dubious whether you will like civ4. what you ahve to understand is that civ4 is much more complicated, longer, harder and in depth than civ rev. but ultimately, i find it to be a more rewarding game. according to lots of people who play civ4 around here, they don't like civ rev: they say it oversimplifies the game etc.
however, civ4 is the best strategy game ever made so at least give the demos a shot. if you like it, buy the real thing.