Yesterday I installed KDE4 on my Ubuntu 9.04 netbook,
the procedure to install the K desktop environment is very easy.
Open a new terminal
Type the following command:
$ sudo apt-get install kubuntu-desktop
during the installation, a window will appear, and you will select the display manager you want to use. I choose the GDM but the KDM is also good.
If you want to integrate kde applications with gnome applications in the same system menu you can install gnome-menu-extended.
gg1