From bad experience, I can say that while it is most likely doable if everything goes perfectly, all it takes is one snag (ferry, taxi, etc) to miss a flight. I always book the 4:30 pm Delta flight. I catch the noon ferry and have time to have lunch ( I believe it's the Green Parrot?) right on the STT waterfront and catch a taxi to the aiport. Get in late, but it's worth it to not add any stress to a long day. Just MHO.