They're about 45 minutes from each other by public transportation (assuming everything goes smoothly, but it's Boston, so that is a poor assumption). If you're taking the T (subway & bus), leave about an hour. If you're taking a cab, it shouldn't take more than half an hour.
The interview ends at about 3:00, but the end is just lunch, a student-given tour, and a wrap-up by the dean (who is very cool and understanding, so if you say you have to catch a flight it won't be a problem). If it's possible to push back that 4:25 flight at all, it would probably be a good idea.
Good luck! Let me know if you have any other questions about Boston or BU.