Meanwhile for Slackware, KDE is still considered unstable for now and *MAY* not be included in the next Slackware release, which will be 12.1 (no exact date though). I think it will be included in the next major release (13.0 my guess).
BUT..... if you have a testing machine, and very curious about how KDE 4 looks like, you can grab KDE 4 packages from Robby Workman's site
PLEASE NOTE: This is a big efforts, as you will have to remove all kde apps and every package related to KDE, mostly QT and the compile all of KDE apps and the dependencies (there are a lot of them and you must compile them in order to work perfect) before you can enjoy them (Robby has managed to provide the tgz format though, if you are too lazy to compile KDE, but you have to use machine running -CURRENT for this). PLEASE READ the _README file first. Take your own risk by using this package as it has been stated:
This contains new and *very* experimental build scripts for KDE4. From what I understand, kde-4.0.0 is not intended as a replacement for the stable kde-3.5.x series; rather, this is more of a "preview" release mainly geared for interested users and developers of add-on applications.Here's the build order for dependencies:
1. boost 2. qt 3. eigen 4. clucene 5. raptor 6. rasqal
7. redland 8. strigi 9. soprano 10. qimageblitz 11. qca
12. qca-$plugins 13. optional/*
I personally would rather to wait for the official package to be released in Slackware.
Read also the discussion on LQ. Robby said that Pat would push some more official packages after he came back from KDE Release Events (That's a good news), but i think it would be in /testing rather than in the main tree of /kde)