The following is written with the conscious realization that I'm not a KDE
dev, just a (rather avid power) user...
Preview release? KDE 4.0 didn't have anywhere near full feature or
functionality implementation yet, so an early alpha or an early technology
4.1 has /most/ functionality implemented, but not all (no proxy support in
a full release?? you ARE kidding, right?? unfortunately not, so not all
even sanely expected functionality implemented even with 4.1), so it's a
late alpha or a late technology preview.
So it'll apparently be at least KDE 4.2 before we even get sanely expected
features implemented and at least generally working (if not yet bug free),
which would make it finally beta quality.
Again, all this said as a very avid KDE 3.x power user, because nothing
else fits my power user needs as well, an avid power user who fully
expects KDE 4 to get there and then surpass KDE 3... some day. However,
that day isn't today, and apparently isn't going to be any time in the
immediate future, certainly not 4.1, but possibly 4.2, and hopefully by
4.3 at the very latest, for rough parity. If they stick to the six-ish
month 4.x release schedule, that'll be Q1 or Q3, next year.