Well for starts why not post your XF86Config-4 file - maybe someone can spot an error. Also, I presume you are using KDM still. KDM has a default directory where it stores files that actually launch your preferred desktop for you. If there's an entry in kdm there's a file for it (probably in /etc/kdm/?). Sorry I'm not more help but I prefer gdm myself.

As always try man kdm

Another thing, see if XF86Config-4~ exists. That would be a backup. *HINT*

Shame on you for not doing:
cp /etc/X11/XF86Config-4 /etc/X11/XF86Config-4.good
BEFORE editing that file. Of course you can always boot Knoppix and replace that file with a known working verison. Keep in mind however that it isn't always so easy to replace a file once you change it.

Have you tried logging in as root as well as user?