I've not tried Gentoo before, so I can't really say anything about it. But I would definitely recommend Slackware from what I understand of your needs.
By default, it comes with gcc 3.3.6, but I believe a 4.x package is supplied as an alternate. Or it shouldn't be any problem to simply compile the new version yourself; it worked for me, anyway. Furthermore, the default install supports almost all (if not all) of the gcc languages including Ada, Fortran, and Objective C. Slackware also features KDE for your desktop, and generally provides a simple, powerful, and stable environment, even by Linux's high standards.
FreeBSD is also a superb operating system (in fact, I like it even more than Linux; but it lacks good support for certain software). It comes by default with gcc 3.4.4, but you can certainly install other versions as well. The default, I believe, must be kept around for system upgrades because of compatibility. As far as KDE on FreeBSD goes, I don't know much about it (I'm a WindowMaker guy), except that it has it's own project page:
http://freebsd.kde.org/
I'd give Slackware a shot, if you haven't done so yet. In any case, with enough searching you're sure to find a distro that perfectly suits your needs!