I just got Civ IV in the sale. I chose it over Civ V based on a couple of obscure things that came up in the few reviews I checked out.
I recommend it, although I don't know how much longevity it will have for me. My current game (my first in Beyond the Sword) has 72 elapsed hours and is only up to the 1930s (ie. more than 100 turns to go unless someone wins before that). Although I might've gone away and left it on some of that time...
For those that don't know it at all, it's a turn-based, grand-strategic empire-builder.
For those that do know it, I'm in 1936 as Japan (random leaders). Single-player, low (chieftain I think) level difficulty, the six AI empires were:
Pericles of Sumeria
Mao Zedong of Portugal(!)
Queen Victoria of Spain
Zara Yaqob of Native America
Boudicca of America; and
Montezuma of the Holy Roman Empire(lol)
(Couple of amusing ones there)
'America' is between me and Pericles, Native America and Montezuma, so I just have to worry about it, Mao, and Victoria. I hadn't had a proper war until 1878. Native America is my vassal state. There is only one continent on the planet.
Neither it nor America was spending much on defence. So when Victoria of Spain moved in on America, she looked like cleaning up. I felt I had to intervene to avoid being surrounded. Over 27 years I reduced Spain to 3 cities, then made peace (1905). I then had five Spanish cities, and two American ones - one came over to me peacefully, and one I got from Spain.
The two American ones went back to America, one of the Spanish ones asked to go back to Spain, and another went into Spanish revolt. So in 1924 I moved in on the rest of Spain. When it was down to its last city, my vassal finally joined in and took it.
Spanish revolts don't happen anymore! (It's a bit of a bug that, as soon as Spain ceased to be, so did Spanish cultural identity, and thus Spanish revolts etc. Not that I'm complaining...oh wait, yes I am!)
I have nearly triple the victory points of my nearest rival. My military could probably take Mao, Boudicca and Native America combined right now (I currently have the only tanks, planes, destroyer and carrier). But I'm too much of a pacifist, or lack sufficient provocation, so the troops are all sitting around twiddling their thumbs and eating my gold.