The second benefit of well-made roundabouts is that they impede head-on and T-bone collisions. All accidents become side-by-side accidents.
Depends on the size of the roundabout, some are designed for 40 mph others for a more sane 20 mph.
I mean, a roundabout is a system for efficient "giving way". 20mph is only "more sane" when a roundabout is in a low speed limit area. We've got plenty of them on 60mph zones - you've got no interest in having cars slow down and accelerate again when not necessary (if nothing else because it's a waste of energy as well as time).
That's not entirely true, with half decent vision I can drive straight onto and off the roundabout again. Certainly in a 30/40mph limit I don't have to slow down particularly. The point of roundabouts isn't to slow people down, it's to keep traffic moving efficiently and safely.
Four way stops are just obviously an awful solution that I've never understood