This happened because some applications (namely ark and nepomuk) now depend on a new package in extragear/libs called libknotificationitem.

I’ve made the necessary modifications in gentoo’s kde-testing overlay to fix the issue  by adding a new ebuild for libknotificationitem and adding libknotificationitem DEPEND lines in ark and nepomuk. I also had to revbump plasma-workspace to r1 and block the old ebuild because libknotificationitem had file collisions with it.

So, if you had issues while @live-rebuilding kde4 today, sync your overlay and update

update: libknotificationitem just got moved to kdelibs/experimental