One comment about a 4 wire setup. Take the panel cover off of your electrical service and you might see that the white neutral wire is on the same lug as the ground wire. Further more only 3 wires come off the transformer that feed your house, 2 hots and a ground, just depends where the neutral and GND are bonded together at. Separating the neutral and ground wire I think is for the GFCI to work better in the event of an internal fault. GND and Neutral are the same potential separating them adds to better user protection from shock.