I have a kde3 app that needs kfmclient to launch a webbrowser, but if I install kdebase3, it will launch konqueror-3, while I of course want konqueror-4.
#!/bin/sh source /the/kde4_env konqueror $*
chmod u+x ~/bin/konqueror-4
keditfiletype text/html(in your kde3 env) and add konqueror-4. It will create a ~/.kde/share/applnk/.hidden/konqueror-4.desktop which isn't seen by kde4 if you use a different KDEHOME for kde4.