Two days ago in a Release Update Marc Brockschmidt announced that Debian Lenny will be released under the number of 5.0, he also announced that it could be shipped with KDE 4, as he says

There may be a possibility to include KDE4 in lenny. The efforts on KDE4.1 have been quite promising and seem to be leading to a desktop environment which can fully replace KDE3.

If you want to check the release schedule of Debian Lenny read here