I don't see why they would have changed the jacks location for a Mark IV as far as input and output goes, so if it helps, the Mark IV reverb input goes to the jack closest to the transformer. Then that white input cable goes into the input of the tank. In the Accutronics reverb tank that is in my Mark IV that means the left (closer) jack. Then the gray cable goes to the ouput jack that is the jack that is closer to the front. From there, the cable goes to the output jack of the reverb tank that is on the right (further away).