I'm a Mac user, but I'd like to get my brother a copy of Civ 4 for his PC so that we can play each other by e-mail. Are the two compatible?

Don´t know. Please post your threads in the right forum. You are in the Civilization Revolution forum. This is a game for consoles, not for PCs or MACs. CIV4 has its own forum.

Good luck. Hopefully you can play against your brother.