If you want to count on "good walking around weather" in February, you shouldn't go to either Lisbon, Amsterdam, or Bruges. There's a reason those airfares are low.
If you can handle possible rain/snow, and are willing to do what you do in New York in February when the weather's bad (bundle up and keep going), then you can visit either one. If you're envisioning pleasant strolls, pick a different month.
I also agree that with only four days, I wouldn't try to combine Amsterdam and Bruges. If you're flying to Amsterdam, there are lots of closer day trip options, from small cities like Utrecht to rural areas. If you're flying to Brussels, in addition to Bruges you can easily see Antwerp, Ghent, or Brussels for part or all of a day (frequent inexpensive trains connect all four).