Yup,
If you have a Mac (like me), you can play vs PC's by using the CIV-4 In-game Multiplayer menu, which uses GameSpy. It seemed sufficiently busy to me, plenty of games.
Make sure you have CIV-4 version 1.61 rev B
If you can't connect: use a more unique username for your Gamespy account, and check your firewall. Or, read the forums for more detailed tech help.
Or, you can download and install GameRanger and play vs Macs (only, I think).
However, it seemed really quiet to me, mostly Age of Empires III (YMMV).
I'll be hosting game on and off, Terra, Small, Monarch, Normal speed, with turntimer. You're welcome to join me.
cheers

-- Eddy