Florence is incredibly dense with important sites for Renaissance art. Here are some of my must sees in order of importance: Brancacci Chapel (best thing there IMHO), Uffizi, PIazzale Michelangelo for the great view (your holiday card shot of the family) with San Miniato church nearby, Academia (David), Bargello, Santa Croce and Santa Maris Novella, the Baptistry, San Marco, Climb the dome of the Duomo, Ponte Vecchio, Palazzo Pitti and Boboli Gardens -- and there is a lot more than that if you have time, which you don't e.g. San Lorenzo, Medici Chapel, Santo Spiritu etc etc. 4 nights is 3 days -- that is barely scratching the highlights. If you feel that two days is enough Florence for you then a day trip by bus to Siena would be marvelous. About an hour; the bus leaves you in the historic center and this is an amazing place. The cathedral is jaw dropping, the Campo town square is arguably the most beautiful in Italy and climbing the Torre Mangia is memorable. https://janettravels.wordpress.com/2011/09/18/climbing-the-torre-mangia-at-67/