Apparently the Civ 4 has a timer in it to lock you out after you've played for the amount of time you specify :-)
I never played Civ III but loved Civ II and IV

i played civ II a lot back in the day, civ 3 a bit when i borrowed it from a friend but then civ 4 came out and it blows civ 3 away in my opinion, if you have a good computer that is, otherwise the Ai turns can drag on and you need to downgrade the visual effects a bit and most importantly limit the number of AI nations and size of the map,
i like playing on a fan made Huge Earth 18 civs(in original start positions)
civ 4 has religion and corporations that i think are not in civ 3(apart from more civs, some different building, units etc) religion is great(however the makers were too scared to give different religions different traits on your nation so they wimped out and made them all the same) and corporations (somewhat basic and simplistic in civ 4 but moving in a good direction and i look forward to them working on the idea for civ 5)

kestas, i never turn that timer on(and didnt tell my gf how to either!)

civ IV is better than 3... for all the reasons sean said... just a phenomenal game all around.

I wish I had the time and the computer to play it for a lifetime... i never get tired of it.

I recently picked up revolutions for the DS. It's a good experience.

I like IV, but I don't think it's THAT much better than III. It seems to be much shorter and less epic in scope. My Civ III games were always enormously long and filled with epic wars, settlement races and the like. Civ IV seems to add complexity and reduce longevity. Still, I love both games.

Sorry to double post...why am I in italics?

I agree with Hereward. Civ IV isn't as cool. Civ III is more elegant, because the board looks like just that, a board, or a map, and there are less features in general. But there is also immense complexity that arises from playing lonnnnng games. Like the one I'm involved in now. It devolved into a Cold War, me controlling one continent and the AI with the other, both of us amassing nukes.

The other thing that pisses me off about IV that I can never ever get over is the railroads. Railroads are 0 turns. There's no explaining that away that's a huge pain in the ass.

Hereward:

You see your own posts in italics to difference from everyone else's.

I recently acquired CivIII. I've gotten really into playing a downloadable Conquest called Age of Imperialism. It's insanely fun.

Thanks Xapi.

@ Thucydides - I had a similar situation on mine, I conquered my continent and ended up having a Cold War with an alliance of Civs on the other, mine involved aircraft carrier raids and Marine landings on disputed islands. It was fantastic. I had to form an armoured rapid reaction force and create strategic airbases for defence, the works. I haven't really had that with Civ IV, it just doesn't seem to produce those situations as much.

Have any of y'all played the Rhye's and Fall series?

Particularly in Civ IV. Impossible to win the Historical missions :) but very fun and a cool simulation of the tides of history as civs break down and new ones appear.

I love Civ III! which is your favorite Civ? Mine will always be the Immortal making, Industious and Sciencific bad-asses know as the Persians