"In the beginning the solution -- for the KDE project, and for
distributions that ship with KDE, such as Kubuntu -- was fairly
simple. Offer both the 3.5.x and 4.x versions, either as a
installation option, or through repositories.
"At some point, however, a disconnect has to come. Celeste Lyn
Paul, a member of the KDE Human-Computer Interaction group, talks a
bit about the decisions Kubuntu had to face as Hardy (and its 3.5
desktop option) gave way to Intrepid's 4.1.x only environment."