I agree with everyone else that 4 days is a nice vacation as my long weekends usually are only 2 days. On my european weekends, I usually leave Fri night, arrive Sat. mornings and then leave on Monday morning to arrive on Monday morning.
Really enjoyed the following destinations so far:
Brussels
Amsterdam
London
Madrid
Next weekend have a weekend to Lisbon planned which seems like a good choice although no direct flights from NY.