Really, portupgrade/portmaster/portmanager will figure out what to upgrade and in which order. This wheel has already been invented several times over. Something like 'portupgrade kde4' will take care of everything. It is useless to keep track of all the ports that kde4 depends on, and all the ports on which it depends. If you really want to get a feel for how pointless that is, try:
cd /usr/ports/x11/kde4
make pretty-print-run-depends-list
make pretty-print-build-depends-list
or
pkg_info -rR kde4_your_version