Pisa, Florence and Siena in 4 days
Ciao Sunny!
4 days isn't a lot of time - the most important thing to know is whether you plan on renting a car to drive around or not, since it will make a difference in what you can see and do.
Let's assume you don't rent a car at all.....
Since you arrive in Pisa at 9.30pm, I'd recommend you spend your first night there and then spend the next morning exploring the
Piazza dei Miracoli and even climbing up the Leaning Tower of Pisa - it is an incredible, unique experience! (you can
buy tickets ahead of time here)
Then you can take the train from Pisa to Florence (takes 1 hour) and use Florence as your base for the next 3 nights. We really recommend spending at least 2 full days in Florence to see all of the main sights - you can see what they are by reading
this itinerary.
Then spend the other day in
Siena.... here is an outline of what I'm suggesting:
14th evening - arrive in Pisa, sleep there - check this category to book hotels in Pisa for your first night:
http://www.tuscanyaccommodation.com/pisa-accommodation/
15th - see Pisa, then move on to Florence in afternoon - use as your base for next 3 nights
Recommend you stay in the historical center so that you are near all of the sights - see this page for ideas of where to stay and to book:
http://www.florenceaccommodation.com/florence-historical-center/
16th - spend the day seeing Florence
17th - Siena - take SITA bus to Siena from Florence - we highly recommend buying the Siena SI Pass and go into the duomo, the baptistery, crypt and opera museum from which you can enjoy a stupendous view of Siena. Of course, visit the Piazza del Campo, if you have time, climb up the tower del Mangia over the city hall to look down on the Piazza del Campo - great great views! Here is
an itinerary with a few more ideas of what to see and do.
18th - Florence - see more of Florence, already be in town to leave this evening.
Sorry it took me a few days to post, I was already writing my reply last night but it was too late!

Hope you have a great time in Tuscany and if you need more specific suggestions for where to stay, let me know!