I won't quote smitty, AMS, or Jethro, you guys can see their answers easily enough.
If I understand Robert's scenario right, he has 1- 2x12 cab at 8 or 16 ohms, and a 4x12 that goes 4 and 8 in the stereo or mono settings.
Maybe I can speak of my own experiences and y'all can clarify or clean up anything I say that is not accurate.
Let me start with, I have 0 0 0 ZERO, experience with a 2x and 4 x cab run together. BUT, I do have a particular experience with 2 4x12 Stereo/Mono cabs of different manufacturers. In this case, I have a Marshall 300 Watt G12/ 75 loaded cab that is operated as per these instructions.
https://www.marshallamps.de/fileadm...nuals/Marshall-External-Cabinets-Handbuch.pdf
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NOW for where I learned that " guessing" is not a method. To ensure safe operation, one must confirm with a Meter the true output results. So follow me here. My 1960A cab is either 4 ohm Stereo, or 16 ohm Mono. To save from having a mismatch scenario like Robert did, I put BLUE Painter's tape over the 4 OHM Stereo side jack, and put the switch to mono 16 ohm. No possible way to plug a 16 ohm load into a 4 ohm cab setting that way.
Also as many already know, I bought a 100 Watt Marshall head and an 4x12 Behringer Stereo/ mono cab that had 4 new Eminence Governor 16 ohm speakers wired into it. So by all logic, 4- 12 inch 16 ohm speakers wired up, should yield what the cab says it should, 4 or 8 ohms this time.
Since my readings while putting my test meter leads at the end of the speaker cable yielded all manner of screwy readings, like one side reading 50 some ohms, and the other like 2,,,,,,,, I figured I better open the patient up.
To make a long story short, I decided to open up my Marshall Cab to do a side by side. I rigged up some alligator clip wires to move the wiring orientations around and check for results. The tests resulted in the Marshall cab 4 - 16 ohm G12's yielded the 4 ohm and 16 with the Marshall Jack.
Now the 4- 16 Ohm Eminences with Berhringer Jack/ switch plate. Taking the Gator clip test leads and wiring it the same as the 1960A, it yielded the 4 and 8 ohm reading as designed and notated on the jack plate. Asking myself how this could be, I concluded it must be a difference in each maker's jack/switch plate. I confirmed this by switching the jack plates from Eminences to Celestians and seeing the ohms swap specs as expected.