ign talked to some of the guys down at Firaxis about Civ IV.
IGNPC: What about the new religion system? That sounds really promising. How does it work in the game?
Barry Caudill: The first Civilization to discover a technology attached to the founding of a religion will establish a holy city for that religion and it will begin to spread, although slowly. To speed up the process, you can create missionaries and send them out to try to convert other cities. Also, just like the Civics, AI leaders may try to get you to convert to their religion.
The game looks promising, and it is expected to be out by this Christmas. It doesn't look to be as big of a change as it was from II to III, but still I am looking forward to it. If you like Civilization, check out the interview.