Since the KDE 4 desktop has been unmaintained for several months and it's becoming increasingly difficult to support two versions of Plasma, we are removing it from our repositories. Plasma 5.5 has just been released and should be stable enough to replace it.
KDE 4 installations will not be automatically upgraded to Plasma 5. However, we recommend all users to upgrade or switch to a maintained alternative as soon as possible, since any future update may break the KDE 4 desktop without prior notice. See [the wiki] for instructions on how to upgrade to Plasma 5.