Nice, I like 4, that's the number we had at my last startup, and it worked well. I figured that the max reasonable team size was 8, but I think your solution (two rooms of 4) strikes a nice balance of noise and allowing the entire team to be very close to each other when they need to be.
Another great environment was inside an old bank vault. Me and one other developer. No windows, two doors, lights down low, thumping techno music. Ah, bliss...