Yes, Civ IV is very good game. In no particular order, very good games include GalCiv II (inc. Arnor), Civ IV (inc. BTS expansion along with some fan enhancements like FFH2 and Revolutions), HOI2 with doomsday, and EU3. OK, actually, I guess that this is the order of my recommended list. I rotate among these games.

Civ 4 is much better than Civ 3. Civ 3 core mechanics was boring, i stopped playing after 5 games (4 of them incomplete). I played Civ 4+expansions for 2138 hours http://www.xfire.com/profile/ellestar/ (about 3/4 of it online) - more than any other game i played in 18 years of gaming.