The past few apt-sources lists do not show xfce4 in the sections. Also they show kde4 instead of kde. This prevents downloading of xfce4 updates and gives a 404 error message (file not found) for kde4. This is easily corrected by manually editing the sections, change kde4 to read just kde and add xfce4. The apt-sources lists should be corrected.
Tim Holt KD7ADG