I've been reading http://techbase.kde.org/Getting_Started/Build/KDE4 and was wondering that if I compile from trunk (using the kde user approach) my libs and apps are overwritten or not when running make install?

If not where is it installed instead. I use my laptop (Fedora 11 KDE4)for productivity too and don't want to mess with my existing installation.

Would be great to know. Otherwise I would install a virtual machine.

Your applications aren't overwritten, if you do everything what you are told in the article. I did a trunk installation like described in this tutorial and this KDE Trunk was installed in ~/kde.
So in ~/kde there is a folder src where the source is in. And the other folders are named lib, bin, include, build etc. like the ones in /.
If you do a slight trick you are able to start applications like KMail which are installed by your distribution (so you have KDE 4.3 AND trunk installed, you can start KDE 4.3 KMail out of trunk).
This trunk is installed as a new user, so you are able to work with your account "me" and if you want to test trunk (or want to develop) you work as user "kde-devel" (that's the name suggested in the tutorial).
I hope it helps you and my explanation wasn't too confusing.
Jan

That's exact what I wanted to know

Thanks

No problem, glad to help you.