Long ago KDE 4.0 was available for those who wanted to try it. Now KDE 4.3 is coming soon and KDE 3.5 will become unmaintained.
However, if you had KDE 4.0, it might have left a symbolic link behind which will break KDE 4.3 when you try to install it. To prevent this, delete /usr/share/config before installing it.
If you don't, five packages will refuse to install which will leave you without a GUI. You will then have to boot into failsafe and do the following:
sh-4.01]#rm /etc/kde
remove symbolic link '/etc/kde'?:y
sh-4.01]#rm /usr/share/config
remove symbolic link '/usr/share/config'?:y
sh-4.01]#apt-get --fix-broken install
You should then be able to reboot into KDE4.
I've reported this as a broken package but just in case it isn't fixed when you want to install it....
(I can't remember exactly what the single-user prompt looks like so don't worry if what's to the left of the hash isn't quite what I've typed.)