The first Release Candidate of KDE 4.0 has been released today \o/. The most important and interesting packages are already available from experimental. If you want to try them, point your sources.list to experimental and install kdelibs5 kdepimlibs5 kdebase kdebase-workspace kdebase-runtime extragear-plasma kdegames kdeutils kdegraphics kdeedu. While I write these lines, kdebase-workspace and kdebase-runtime are stuck at NEW, so you can fetch them (actually, all the packages), adding to your sources:
deb http://people.debian.org/~ana/rc1/ ./.
These packages are not co-installable with current KDE 3.5.8, so if you want to test them, do not use your production environment and use instead qemu or virtualbox-ose. Even if the release is named RC1, this is not yet a fully usable desktop, but you have as well the possibility installing KDE4 apps on a your KDE3, i.e. try okular :)
If you are lazy, or not in hurry, you can wait a few days until the Debian KDE4 Beta4 Live is ready!
Finally, please, do not report bugs against KDE4 in the BTS yet, if you find bugs on KDE itself, report them to directly to upstream in the KDE bugzilla, and optionally you can note down it in this wiki page.