HD2: 1280x720 mirrors, each mirror produces 1 pixel displayed on the screen yielding a 1280x720 final resolution; pixels are rectangular shaped and don't overlap, individual pixels are visible on the screen.
HD4: 640 x 720 mirrors, each mirror quickly moves (the so-called wobulation) to produce 2 pixels displayed on the screen yielding a 1280x720 final resolution; pixels are diamond shaped and slightly overlap, producing a smooth "pixel free" image - refered to as the smoother more cinematic effect.