Tina,
Not much should happen. It should be able to open psm 4 documents without any trouble. If they were made on a different machine, the old printer drivers might not exist anymore, so you may need to reselect a new printer and reconfigure finishing, printer settings, etc... You might also get a couple warnings about the document having been made in a previous version and not being compatible, but don't worry about those.
All in all, it should work without much issues.
Regards,
Raphaël Lalonde Lefebvre