I have written a small HOWTO (in Indonesian) to upgrade KDE 3.5.10 to KDE 4.2.0 on Slackware. This is a compilation between Patrick Volkerding's README and what i experienced when i upgrade both my laptop and my desktop at my office. I have added some notes based on what i see on my system after i upgrade to KDE 4.2.0. I'm still using KDE 3.5.10 on my desktop at home due to insufficient space.
Please note that KDE 4.2.0 provided in Slackware-Current is supposed to be installed on top of Slackware 12.2 at minimum. Do not try to install on previous version of Slackware.
In the last two days, i have been playing with KDE 4.2.0 and so far, it's just great.