KDE 4.2.3 has been released and packages are available for Kubuntu 9.04.
To install it add this line to your software sources.
deb http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu jaunty main
It is strongly recommended that you verify the integrity of these packages by installing the archive's GPG key. You may do this by saving the PUBLIC KEY BLOCK available from this page to a file and importing it from the authentication tab in KPackageKit's software sources window.
Then refresh and do a full upgrade.