Hi I would let TD take care of it as an Add on.
Enable the rebuys
disable rebuys after round 4
enable addons
set maximum rounds to an unachieveable level
set maximum Add on to 1
That should give you your desired result.
(http://thetournamentdirector.net/forums/index.php?action=dlattach;topic=1831.0;attach=1383)