Upgrading to gcc 4.0

Hi,
 I am wondering how to upgrade to gcc version 4.0 or later (without
breaking
anything of current version), if I currently have Xcode 1.5 and gcc
version
3.3 on os X 10.3.9

I looked around for it, and could not find a solution for it for OS X. I
would like to install it (in a standard location preferably) in such a
fa****on, that I can always switch back to using version 3.3 in case of
problems.

Thanks,
-amit.