A day trip to Santorini really isn't practical; if it could be done, you would probably spend more time in transit than on Santorini. Keep in mind that the 4-hour time period that might be available will include the time to get from the Santorini ferry port up to Fira, and of course back to the port for the return trip. Travelling to and from the port will eat up a good chunk of available time. You would wind up with substantially less time on Santorini than cruise ship passengers typically get to experience -- and Santorini deserves much more than just a couple of hours.
If you really must visit another island, take an excursion to Delos or do a daytrip to Tinos, which is only a short ferry ride from Mykonos