Even though KDE team has just released KDE 4.0 for public use, it doesn't mean that they are satisfied with it's results. Most people agreed that KDE 4.0 is more suitable for developers to test their application whether it works or not with the new infrastructure of KDE 4. It also gives developer time to adopt or port their application to work with KDE 4 libraries which has changed quite a lot.
From Phoronix, i got this KDE 4.1 release schedule:
January - KDE 4.0.1
February - KDE 4.0.2
March - KDE 4.0.3
April - KDE 4.0.4
May - KDE 4.0.5
June - KDE 4.0.6
July - KDE 4.1.0
August - KDE 4.1.1
In my opinion, Slackware will try to include the KDE 4 package after it has released their first maintenance update (the bold one) since most of the problems found during public testing will be found and (hopefully) has been fixed in their regular maintenance version of 4.0.x version that will be released each month. It's too risky to include them now (probably on /testing) since it requires a lot of changes to be made before you can enjoy them.
For the -Stable version of KDE, next month is the month where KDE 3.5.9 will be released, so i guess we should stick with this version for now (unless you want to start hacking your system to use KDE 4)