sudo conary update group-kde4=kde.rpath.org@fl:2-kde4.2-devel
and it download all requires to use kde4.2.
Kde 4.2 its not official of Foresight so if you have question about it. you can Visit:
foresight-kde channel on irc.freenode.org and ask for how use it.
update: *** Reading foresight channel you must migrate your foresight system to 2-qa for install kde4.2 because deps of python2.6 and python2.6 for now its on 2-qa branch and 2-devel