4 days in Tuscany- recommendations?

You will see open rolling hills all over Tuscany.

It sounds like you will have a car, so I'd recommend establishing a base to explore from. You could actually just stay in Florence (which is the capital of Tuscany, not separate from it) for all six days, which would give you plenty of time to do day trips and still experience all there is to see in Florence. Or you could do 2-3 days in Florence and 3-4 days elsewhere (I'd recommend Siena, because it's a great town to come back to in the evening and there's lots to see and do there).

Personally I found the area south of Siena (including Pienza, Montalcino, and other towns in that area) to be a tad more scenic than the area between Siena and Florence (including Volterra and San Gimignano), but it's all beautiful, and each town has a different character.

In terms of popularity among global tourism, the most popular towns in Central Tuscany (besides Florence, which is Tuscany's capital city) are:
Siena, San Gimignano, Chianti Hills, Volterra, Pienza, Monteriggioni.

Of course there are dozens more, but the above locations are the top hits in the mass tourism industry.

If you have 4 full days in Tuscany and have a car, there is ample time to visit all of the above.
1. From Volterra area, on the same day, you can very easily visit San Gimignano and take a drive through the Chianti Hills starting near Castellina.
2. You can devote another full day to visit the Val D'Orcia area (Pienza, Montalcino, Montepulciano, Buonconvento, San Quirico)
3. Another day you can devote fully to Siena, with Monteriggioni visited on the way back from Siena. One hour is all you need in Monteriggioni and you have to pass by it to get to Siena.
4. The day you visit Volterra (which takes just a few hours) you can relax at your B&B.
You can do the above 4 options in any order you wish.

Day Trip 1:

Day Trip 2:

Day Trip 3: