2-quadrant device will draw energy from the supply to run the load either forwards or backwards

4-quadrant device is also capable of returning energy from the load to the supply. This normally only applies to motors, where it is feasible to see the motor returning energy during braking. (in DC systems if the load is a battery or supercapacitor storage system, that can be 4-quadrant too)

A 2-quadrant system only works in quadrants I and III