I gave "/testing" a higher priority in slackpkg.conf
Code:
# The lines below will set the download priority.
# Default values: /patches /slackware /extra /pasture /testing
#FIRST=patches
FIRST=testing
#SECOND=slackware
SECOND=patches
#THIRD=extra
THIRD=slackware
#FOURTH=pasture
FOURTH=extra
#FIFTH=testing
FIFTH=pasture
Edit: Looks like this is no longer needed.
KDE3 is gone. KDE4 has been moved from /testing,
and is now an official part of Slackware-current!