Hard to give good answers without more context. Have you been to this region (or to Europe) before? Is this part of a longer trip to Europe (if yes, where/how long)? What are your likes/dislikes and priorities?
Most importantly, how exactly are you counting your "4 days" there? Does that include the days you are arriving to Amsterdam or the day you are departing? Where will you be coming from and going to (via train to another city nearby, or flying to/from another continent - makes a difference)?
Without that, hard to know how much actual, usable time you really have, or whether a day trip (to anywhere) makes sense. Bruges is great, but...a daytrip puts you there when it's most crowded and depends on how much time you really have in Amsterdam (and how you would prioritize things, which is different for everyone).
FWIW, answers to all your questions (along with good suggestions for making the best use of your time) can be found easily in the Rick Steves guidebook for Amsterdam.