First & Last (at the harbour) used to be a good place to eat, but that was a while ago.
You can almost walk round the island in an hour, no need to get a vehicle.
Fort Clonque is worth a visit, but if you're not actually hiring the place you might not be able to persuade anyone to let you in.
So, you can walk from the airfield to the harbour, have lunch, walk round the coast to Clonque (the short way), and back to the airfied in your four hours. Sorry, I don't remember the exact route across the fields from Clonque back to the airfield but the path should be obvious enough.