640 inch - pounds
The spring constant is 40 pounds per inch because 80 / 2 = <<80/2=40>>40.
It takes 160 pounds of force to stretch the spring 4 inches because 2 x 40 = <<240=80>>80 and 80 x 2 = <<802=160>>160.
Work is force times distance, so the work done is 640 inch-pounds because 4 x 160 = <<4*160=640>>640