Those tips are also suitable for Kubuntu.
First of all,we need to add source for KDE4,as I guess most of sources you are using still haven't supplied KDE 4 packages.
sudo gedit /etc/apt/sources.list /* open sources.list file
then,to add below sources into you sources.list
deb http://ppa.launchpad.net/kubuntu-members-kde4/ubuntu gutsy main2
Save and exit.
Updating your sources by running: sudo aptitude update
Lastly,Intalling the KDE4 by running:sudo aptitude install kde4-core
NOTE:If you ever installed KDE4 before,you need to delete some files installing KDE4 in order to avoid confict before :
sudo aptitude remove kdelibs5 kde4base-data kde4libs-data3