well, since KDE 4 is quite young and unstable, with many little things that doesn't work just right, I prepared a dedicated partition...I installed a debian unstable, WITH GNOME, as a back up (funny, no, gnome to help KDE ?) and then KDE 4 on experimental...works quite good...
Black screen I had too...what I do is a complete reinstall of the system, and then, some things go better and other fails, ha!
Give more details, your general setup..I tried KDE 4 on Suse, one click install, nice, but since I'm a convert form Suse to Debian, I can't stand the slow slow suse...
Buntu, Okobuntu and zamobuntu..sorru, notu foru me!
Anyway, have a google look on that balck screen of death..Make sure your xorg.conf is well configured, I took away most of my problems when I rewrote it by hand, to have accelaration, modules, and the like...
KDE 4 is nice, would like to have more style there...
See you