Don't underestimate the differences between OS X and Linux. Moving Civ4 from OS X to Linux would not be a trivial porting task. There's a world of difference between the APIs (programming interfaces) for the display managers in Linux and OS X. Sure, you can use a lot of Linux software in OS X, but only because it either doesn't need a user interface, or it runs in X-Windows, or someone's done extensive work to move the UI into Aqua.