Ubuntu Geek posted a guide about installing KDE 4.0 (Stable) in Ubuntu Gutsy
KDE 4.0 is the innovative Free Software desktop containing lots of applications for every day use as well as for specific purposes.Plasma is a new desktop shell developed for KDE 4, providing an intuitive interface to interact with the desktop and applications. The Konqueror web browser integrates the web with the desktop. The Dolphin file manager, the Okular document reader and the System Settings control center complete the basic desktop set.Howto Install KDE 4.0 (Stable) in Ubuntu Gutsy