I don't recall seeing GCC updated in a distribution unless it is a minor revision. Upgrading to a major revision can cause incompatibilities for the existing software. Based on what I have read it has to do with the Application Binary Interface (ABI).
There are distributions that allow you to do this, Gentoo for example. If you really want to start using 4.2.0 you can do it with Fedora by building in a target directory where there is no previous version of GCC present. Then it would be a matter of using this path to compile/build your software. You can find instructions for this at gnu.org on the Installing GCC