Within the next week, I’ll be changing how slotting works with KDE SC 4 packages in the
kde-base category. All KDE SC 4 packages will be moved into
SLOT=4, from their current slots (which match the KDE SC version). This will cause portage (or your package manager of choice) to take slightly longer on the first sync after the update, but will allow much simpler upgrades between KDE SC versions. As all versions of KDE will now be in the same slot, portage will see a new version of KDE as an upgrade instead of a new slot blocking the old, which will considerably reduce the list of blockers during the upgrade (usually, it will reduce it down to exactly the list of packages that should be blocked anyway).
The reason that we introduced this slotting was to support
USE=kdeprefix, which allowed you to install multiple versions of KDE simultaneously, in
/usr/kde/VERSION. As we can no longer support such installs, the ability to do so has been removed, and with it, the need to have separate slots for each KDE version.