Usually way harder. On Civ 4, I wasn't able to beat the CPU above Monarch level without going for cultural victory, which I think was the easiest for that game. I don't see how someone would achieve a Conquest victory in 4 with the difficulty cranked up. There are so many penalties that it can take the whole game just to conquer 1 of the 6 opponents, unless you are doing something wacky like save/restore to win every fight. I try to only save when I'm quitting for the night.
I've picked up Civ 5 but no chance to try it yet.
Edit: Game is crashing after opening cinematic:
http://forums.steampowered.com/forum....php?t=1439122
...and even if you don't have that issue, there are plenty of others on Steam forums. My impression is that this game is not ready for primetime, though it works for some. Civ 4 was a far more polished product on the day of release.