MesaENGR412 is right on the money.
The "4 ohm" jacks are misleading, and it's Mesa's fault. They are meant to handle two EIGHT ohm cabs, making the total load 4 ohms, hence the misleading label. Two 16 ohm cabs, one in each "4 ohm" jack, will yield a load of 8 ohms total. Normal, no mis-match. Two 4 ohm cabs will bring the load down to 2 ohms, not good. I could be wrong, but I think setting the Marshall cabs to 4 ohms splits the cab into two 2 x 12 sections for stereo, doesn't it?
Page 12 of the Roadster manual:
"Two 4 Ohm, one 8 Ohm and one 16 Ohm jack are provided for speaker interfacing. The Roadster is not very sensitive
to speaker mismatches and will not be damaged by them, except that very low ohmage loads will cause the power tubes to wear faster. A single twelve-inch 8 Ohm speaker should generally be connected to the 8 Ohm output.
When using two 8 Ohm speakers, connect them both to the 4 Ohm outputs provided (because the total load is 4 Ohms in that case.)
(And two 16 ohm cabs equals 8 ohms total. With any two cabs in parallel, the total load is half the impedance of one cab.)
Check out the information further back in this manual regarding speaker impedance and possible speaker hook-up schemes. 4x12 cabinets may be 4, 8 or 16 Ohms. If you are not sure of the impedance of your cabinet, you may need to remove the Rear Panel in order to verify the impedance rating of the individual speaker or speakers. MESA 4x12 and 4x10 cabinets come standard wired to 8 Ohms, and are wired in series/parallel. Some Non-MESA 4x12 cabinets are wired 16 Ohms using four 16 Ohm speakers. By wiring all four speakers in parallel, you can reduce the cabinet to an impedance load of 4 Ohms (assuming the speakers are 16 Ohms each.) No matter how unusual your speaker setup, it is always possible to get good performance."
Hope this helps
