You will probably have to reinstall QuickTime so that it Vegas 4 has access to it. Make sure you install the entire QuickTime system including authoring components and not just the player.
You'll need to install QuickTime 5.0.2 or later, Vegas 4.0 shipped with a new QT plug-in that requires a more recent version of QuickTime. Be sure to do a custom install, select and install everything as more recent versions of QT seem to have more interdependence among components.
You can check your QuickTime version by running the player and selecting Help | About.