one home base for 4 nights in Tuscany?

I would base in Pienza or Montepulciano
4 nights is not 5 days- it is 3 full days

Where and when are you getting the car, dropping the car?
You could visit SG and/ or Volterra on your way to Florence

Or plan to drop car in Siena and spend a night there before Florence

ETA
I see from your other post you are thinking of car from FCO?
Pienza and Montepulciano are both easy drive from FCO
You might start looking for accommodations with parking to see what your options are
Pienza is very easy to drive in/out and there are several hotels just outside walls with parking

Note-Cortona is closer to Montepuciano/Pienza not SG/Volterra
Other towns to visit from Pienza area-San Quirico, Montalcino, Monticchiello, Bagno Vignoni, Arezzo

All the towns you plan on visiting are extremely popular with tourists. If you want to experience a quieter, more authentic Italian village or town, try picking a place you have never heard of. Since you will have a car, that gives you a lot of options. But you also don’t want to spend all your time driving, so I would suggest keeping your day trips to a smaller area.

With just 4 nights, I would stick to one base. Moving eats up time. Of the towns you listed, Pienza and Volterra were my favorites. I found San G. kind of dull. We were able to pick up a tour of the papal palace in Pienza through the TI office. I have no idea if that tour is still available, but it was terrific.
One thing to be careful about. Roads in Tuscany can be narrow and winding. A 25 mile distance may take an hour to cover.

I would suggest basing in Pienza for the simple reason that it's probably the most difficult place (of the towns on your list) to find parking when visiting during the day - although it can be somewhat tough finding parking in Cortona too.

Pienza is really nice in the morning before it gets busy with day visitors, and especially in the evening after they all leave. There are many nice towns to visit within an easy drive from Pienza, east or west on SS146, or south on SR2.

We did Tuscany and stayed in Siena as a base.