You will love the changes in Prov if you haven't been in a long time. Go to Waterfire.org and read about the frequent festival that runs along a canal that goes from the Bay out to Rt 95. That should make a Providence fan out of you! There are a number of "downtown" streets near the big hotels and the Providence Place mall and there is the usual Thayer St stuff around Brown U.
3-4 hours will wet your appetite. I suggest good walking shoes or sandles, park near on of the hotels that you will see coming right up after you exit off Rt 95 then maybe head up towards Brown Univ, or over to Atwells, or onto Empire Street and grab lunch anywhere- tons of choices.