The Slackware changelog:
"The next Slackware
release will contain KDE 3.5.9, but we're targeting KDE 4.1.x for the one
after that. The application end of things doesn't quite fully cover KDE3's
functionality yet, but by then it will. As I'm sure most of you know, Robby
has put up test packages of the initial KDE 4.0 release which I've tested and
found to be consistent with what to expect from a developer's preview."
There is a KDE4 Slackware build out from rworkman that will build KDE4
on Slackware 12.1.