Probably more of a 3.5 star...
This abbey is absolutely gorgeous, and all the grounds are fairytale like. We really appreciated how close it was to towns of Pienza, Montefollonico, Montepulciano, Bagno Vignoni, Montalcino, etc. It is centrally located, and the countryside around it is breathtaking.
The check-in and check-out process was easy and friendly, and staff were very helpful in providing information about local dining, markets and wineries, as well as assisting us with some small needs and requests throughout our stay. We haven't stayed in other B&B properties in Europe before, so we can't compare the breakfast, but we thought it had a lot of variety, with warm options, lots of delicious baked items, so we were happy!
There are some concerns with the rooms - they are pretty bare and basic, so if you are OK with a somewhat ascetic stay, you are in the right place. Our room had a decent kitchen, but all we had for supplies were two coffee cups, two wine glasses, two coffee spoons, a tea kettle and one single pot. That's it! I am not sure why provide a kitchen if it is not stocked with basic cooking supplies, plates, forks, etc..?
The only other major concern was the shower. Yes, it is smaller, like in many European rentals, but the problem was with its fluctuating temperature. The water flowed on a hot/cold repeating cycle, so the only way to take a decent shower was to step under the water during the warm part, and step out during the cold. Rinse, repeat... There must be a problem with the tankless water heater in the kitchen.
The room has fairly cold floors (our stay was in early October) with no rugs, and the extra wool blankets are pretty rough to the touch. It was overall a bit monastic.
But again, we didn't come to Italy to linger in our rental, and its beauty, and proximity to many interesting sites still left us with mostly great memories.