Official Slackware packages of KDE 4.1 (and dependencies) are now in the "Testing" directory of Slackware-current. See the changelog for more information, and read on for a quote from Slackware founder, Patrick Volkerding!
Slackware creator/maintainter Patrick Volkerding writes this in the Slackware changelog:
"Added KDE version 4.1 to testing! :-)
Patrick has included scripts to help upgrade from KDE 3.5.x. Take advantage of the README's and documentation he's included (and notice that this information is for Slackware "current", not Slackware 12.1)
Kudos to Rob Workman for making KDE 4.x available for Slackware for the last few months.