I have played Civ 4 quite a bit and I loved Civ 2, but I never played 3. Just wanting some opinions!
IMHO, Civ4 + expansions is a better civ experience than Civ3 + expansions. Why? As a "builder," Civ4 allows for more balance in gameplay than Civ3. Additionally, a couple annoying features of Civ3, namely corruption & pollution, have been removed. Also, I prefer the restraints in Civ4 that prevent the spamming of cities. I still play Civ3: Conquests now and then in CFC's GOTMs.