At this point, best to wait. The following comments appear in the latest KDE4 base that is available as an extra for F7
This package does not include the KDE 4 versions of applications which are
provided by KDE 3 because of file and configuration setting conflicts.
WARNING: This is a beta version. It probably has many bugs left.
Also, the API and ABI are NOT fully stable yet (soft freeze only).
Note that kdebase4 does NOT include the KDE 4 Desktop (kdebase-workspace, i.e.
Plasma, KWin version 4 etc.), which is still too incomplete and unstable for
daily use and which would conflict with KDE 3.