Civ 4 is a great game they really did this time, great gfx, great soundtrack and great gameplay. The religion is nice improvement and another reason to conquer other civs, another improvement its the costumisable civics. To bad the game goes to fast compared to civ3, even in epic mod it just changes the production needed for each building, but the tech tree still goes quite fast, and it doesn't give you much time to build stuff.
Overall it's very fun to play (like any other civ), and earing Mr. Spock voice with every tech makes me laugh :p