I need to install KDE 4 on Kubuntu 15.04 instead of Plasma 5. How can I do this?
the reply to the bug report linked by @user26687 says it's not possible. Wish it was.
The suggested workaround is to use the long term supported 14.04. Bäh.
A better workaround, which I found in the comments here is to install the gnome desktop until the bugs in kde5 are worked out:
apt-get install gdm