You sure about that Koz? I thought the towing rating was purely the amount of weight the trailer could weigh to be towed safely by the vehicle, excluding the cargo, passengers, etc inside the vehicle. GVWR is the combined weight of the trailer tongue weight plus the vehicle, plus the cargo (for a V6 this would be 5580 = 4300 (weight of 4Runner) + 500 (Max tongue weight) + cargo weight (maximum with trailer of 780 or roughly 4-5 adults)).
The GCWR (which we do not have listed fo the 4Runner) is the combined weight of the vehicle and the towed trailer, but this number is really only tracked in pickups and large commercial trucks.