Play Civ4 at the level beyond which you can win.
Which means you haven't figured out how to beat it. I'm talking about when you find the inevitable holes in the AI so you can beat it whenever you want. You can hobble yourself to try and find some other way to beat it, but the satisfaction is going to go down more and more.
Playing a human, even the same human over and over, is very different. You'll both know strategies that have worked before but, unlike an AI, you don't know which is going to work. You'll need to out-think and second-guess each other. And, no matter what works this time, it's pretty unlikely to work next time. Computers, on the other hand, never learn.
At least not yet. When GalCiv 3 was in development, Stardock was toying with the idea of tracking what strategies were and were not working on a server that would then having the game pull from that server. They didn't do it but maybe someday....