The critics are wrong: KDE 4 doesn't need a fork (ars technica)

Because it wasn't such a piece of crap. That's just negative hype that 
people are repeating after each other, probably because it makes them 
feel better about themselves. Plasma in 4.0 wasn't all that stable. 
(Although by 4.0.4 it was good enough for daily usage, unless your daily 
usage consists of futzing with your desktop settings).

Many parts of KDE 4.0.0 were excellent. For instance, the core libraries 
that people use to build applications against, and some of those 
applications themselves. Okular, Marble, Dolphin.

The release of 4.0 meant that people working on applications outside the 
immediate KDE release cycle could finally really get to work. For 
instance, it was really, really difficult to develop against the shifting 
libraries before the release. The release fixed the libraries and made 
application development actually a lot easier to do.

Releasing a 4.0.0 was totally appropriate and a good decision. If 4.0 
hadn't been released, we'd have been unable to make as much progress with 
KOffice as we have, for instance. KOffice 2.0 still builds against 4.0 -- 
a stable and usable set of libraries.