First update your system.
Press Reload To get current file list
Click Mark All Upgrades
DO NOT USE apt-get update and apt-get upgrade from the command line. This has never been recommended. If you insist on using the command line, the correct procedure is apt-get update and apt-get dist-upgrade. If for any reason the system says it is not able to retrieve a package do you wish to continue always say no, report it to the forum and try to update again later.
Warning - updating to KDE 4 will remove all of your KDE 3 desktop and applications.
01. Turn off Compiz 3D desktop
02. Open Synaptic , click on Settings -> Repositories
03. If belnet.be is checked uncheck and select another repository (belnet.be hasn't been updated in almost a month)
04. Click on new repository you selected.
05. Change kde to kde4 in the sections area (main extra nonfree gnome kde -> main extra nonfree gnome kde4)
06. Press reload to get updated file listing
07. Click on Search
08. Type in task-kde4
08. Select one of the task-kde4 packages (task-kde4) will install most common components (task-kde4-minimal) will only install kdebase and kdelibs for a minimal desktop. Right click on one of the task-kde packages and mark for install then click apply. Warning this will remove all of your KDE 3 desktop and supporting applications!
10. Reboot system. (If you get dropped to console hold alt and control keys down then press delete key to reboot system.
After rebooting log in to the new kde desktop. The KDE 4 desktop is still under development and while fairly stable many 3rd party supporting applications have not been ported to KDE 4.