As far as I can understand from all the conversation, 4WD is good only for starting the car. It is as good as 2WD in stopping the car. The feature that helps a lot in braking is ABS. I'm not sure why Toyota decided not to enable VSC in 4WD mode. I would assume the ABS is always active regardless of 2WD or 4WD.
Having said that, I would leave in 2WD if it becomes FWD. Otherwise, it's a 50-50 situation.