4 days in Florence will pass quickly, particularly since your arrival day is one of them. I personally don't think it's worth a detour to Pisa, and not worth day tripping from Florence at all if it's your first time there.
Look at it this way:
Day 1: Arrive, check in, use the afternoon to do a walking tour. Dinner. Bed.
Day 2: Visit Palazzo Pitti / Giardino Boboli in the morning. Lunch. Museum, relax. Dinner.
Day 3: Accademia (buy tickets in advance or book a tour to cut the line). Lunch at Mercato Centrale or nearby. Afternoon, shopping, Duomo/Campanile, maybe the Natural History museum up past the Accademia (its small). Dinner.
Day 4: Uffizi. Lunch. Uffizi again? (It's big) or Museo Galileo nearby. Shopping, walking. Dinner.
You could have another trip there with a completely different itinerary.
Then on to Rome, where 5 days will also go quickly. Same with Naples.
If you're a first timer, you're better off planning fewer primary destinations - maybe 3 cities over 2 weeks - and if you truly get bored in any of them, you can pick a day trip and go. It's better to do that than to plan to re-pack every 3 days on a 16 day trip. It seems like a long time, but each day you transition from one place to another, you're going to lose half a day.