So I'm trying to download Xcode 4 for Snow Leopard, and it appears to me that you can no longer do so without upgrading to Lion? Is there some way to get this without having to upgrade my entire operating system?
It was only available through the App Store as a paid download for Snow Leopard. If you can't see Xcode 4.0.x there still in the App Store, then it has been pulled and is probably no longer available.
Your only other option might to be become a registered Apple Developer, you might be able to download a DMG of 4.0 then, but that's a shot-in-the-dark untested solution.