KDE 4 debuted this past week. Details are just coming out and, frankly, this looks like the spectacular and powerful desktop Linux has been in need of.
Version 4 has been redone under the hood and in all the visible spots. While not all new, enough of it is that just mentioning it all and tossing in some nice images takes 7 pages in the ARS review. The new version is buggy and not quite feature-complete, though it looks to me worth installing and working with right now. For that reason version 3.5, the current stable release, is still fully supported. I suspect it will take a while for the various distributions that offer KDE to switch over. That's okay.
The review is definitely worth your time. It's pretty well written and will give you a great overview of what is to come. As to whether or not you want to jump into version 4, you'll have to decide that for yourself.