If the definition of Civ IV includes patches, expansions, and mods, I think it's not only the greatest Civ ever, but the greatest computer game of all time.
Civ V could easily do worse. I'm excited about hexes, ranged combat, the end of SoD, unprecedented modabillity. I'm concerned about the game becoming less epic and too tactical, day 0 DLC ( and possibly more to follow ) potentially dividing PBEM, GOTM, succession games,etc., and modding audiences.
Hopefully they will have meaningful explanations at E3.