I'm the KDE buildsystem maintainer, and I started to use KDE 4 with version 4.5.
At least for me, the difference between everything up to 4.4 and 4.5 was huge.
On my plain Slackware install 4.4 (was it in Slackware 13.1 ?) I was able to mess up/crash several things in plasma within one hour of playing around, so I stayed at 3.x.
Then, with Slackware 13.37 there came KDE 4.5, and it is *beautiful*, and I can't remember any crashes.
And I can configure everything the way I want it to, more than I could with 3.x.
So, if you haven't tried after 4.4, give 4.5 ... 4.7 a try. IMO it is very different.