Why not just run it in qemu or something like vmplayer? That way if you mess something up the original image is fine no need to reinstall.
Just curious where are you planning on getting KDE4, the newest I know of is the snapshot of 3.80.2.
If you haven't already you may want to take a look at:
"Builds KDE from its Subversion source repository."
http://www.kde-apps.org/content/show.php?content=23840
If you are following the directions from here:
KDE Developer's Corner - KDE Trunk
http://developer.kde.org/build/trunk.html
You "should" be safe enough just building/installing it on your working system.