Civ 4 bored me right out of the gate. I ended up letting my brother have my copy.
The new features seemed more annoying than interesting. As for Civ V, well I'm focused on Elemental at the moment. The setting is more interesting. You can only beat up on the Russians, Egyptians, etc. so many times before it loses the thrill.
Plus, Elemental is easier to mod than I originally expected ,and has me wondering about doing a GalCivII mod now. This is something that I never really felt the need for in Civ (i.e. the units in other unit's clothing syndrome).
If Brad is able to develop an awesome AI for Elemental, well that should be his little trade secret. The Bees will know, though...