The only thing about braking with less than your full hand is that you need to be VERY sure the cable is set so that that the lever will never, ever, ever come to the bar, no matter how hard you squeeze or what you land on. People have lost fingers that way.

The Motorcycle Safety Foundation still insists on full-hand braking, even though you have a LOT more throttle control if you brake only with two, and even though it's really a non-issue with hydraulic brakes. But I'd be a lot more nervous about it with a cable actuated brake, and when you don't have a throttle you need to control, I don't see a lot of reason not to brake with all four.