There's not going to be a civ 5 for about another 20 years or so. Why? Because of the civ on facebook.
Big deal. Games in general have about a 5 year shelf life. So the version at Facebook might run for that long, then they'll be looking for a new source of revenue. But I doubt that the Facebook venture is going to be all that lucrative compared to a real release, and it won't even run for the full 5 years before they start looking for more ways to make some money. I give it about 2-3 years, then we'll be hearing about another Civ, with release a couple of years after that.