* Remove previous KDE 4 packages, they are not compatible (apt-get remove kdelibs5 kde4base-data kde4libs-data)
* Add deb http://ppa.launchpad.net/kubuntu-members-kde4/ubuntu gutsy main to your /etc/apt/sources.list
* Install kde4-core, note that PPAs aren't authenticated so you will likely get a warning when installing
* KDE 4 apps should appear in your KDE 3 K-menu or you can run a full session by selecting "KDE 4" from your login manager.
* To avoid having to start a second X server for a full session install xserver-xephyr and run Xephyr :1 then and run /usr/lib/kde4/bin/startkde in the Xerphyr xterm.