Actually as I write this I am in one of the 7 bedrooms of Villa La Querce. It's about 20 minutes south of Florence in Impruneta. A little bigger than you need, but a fabulous property. Just invite two more couples!
What does your group composition look like--all adults? mix of adults and kids? mobility issues for anyone? You'll have a much larger pick of four-bedroom places outside the city, but getting in and out of Florence can be very unpleasant and trafficky. When our kids were younger, we did both stays in the city and stays in the countryside, both of which have advantages and disadvantages.
2 Apartments in the same building / same floor. I've stayed in both multiple times and would recommend them. The Francesca is 2 BR / 2 BA. The Simona is 2 BR, 1 BA. The owner of both is very good and responsive. You do pay for electricity separately but even in summer running A/C it was not outrageous.