Check the schmatics on this site, remove your chassis and check which color leads are connected to the jacks. If the 4 ohm tap is not connected, you can either rewire it for 8 and 4 or plan on getting a cabinet that is 8 ohms. I suggest going to the 8/4 taps. That way you can connect a 4 ohm cabinet, an 8 ohm cabinet or 2 - 8 ohm cabinets. You cannot connect a 4 ohm and an 8 ohm, when plugging in 2 cabinets, both jacks are connected to the 4 ohm tap.