Ciao Annita,
The car ride from Milan airport (considering Malpensa) to Florence is around 3 hours and 45 minutes.
The train ride from Milano Centrale (so you do need to get there first) to Florence's SMN is under 2 hours, about 1 hour and 40 minutes. So if you do not plan on stopping along the way to see places, the train is convenient.... depends on how many friends you are total, of course the car might turn out to cost less for the drive down than the train, check it out both ways.
The truth is that if you're going to stay in the center of Florence for your 4 days and use it as a base, the car isn't a good idea. Unless you stay outside of the historical center with parking included.
The historical center is a ZTL area, which means a traffic limited area that only those with special permits can enter. Read more about it
here. So if you stay in the center, it is likely the car has to remain outside of the center parked - parking is mostly PAID so you have to consider that as an additional expense.
In 4 days, I would suggest no car and spend 2 full days in Florence, then on another day take train to Pisa and then on to Lucca for a full day out. The last day can be day trip to Siena.
The plan would be similar if you do have a car but stay outside of the center or even in the surroundings, although you won't need the car for the time you spend visiting Florence. Florence is a walkable city as it is quite small, then with cars being limited it is great to walk around.
Take a look at
http://www.florenceaccommodation.com for places to stay - if you do want the car, I suggest staying outside of the center or in the surroundings (where you can just catch a bus to town).