bass cab wiring diagram for 4 speakers

You need 8 ohm speakers. Make two parallel circuits of two speakers. 8/2=4 ohms. When you connect these two circuits in series you get 2*4=8 ohms.

Probably the easiest way to achieve what you want would be to make a two input cab and then use a device like a Radial Cab-Link or Palmer Cabinet Merger.

Cab-Link - Radial Engineering
CAB M


I believe this image can be modified to do what you want. I pulled it from here. stereo/mono cab wiring jacks?


Basically you need to add a second 8 ohm speaker in parallel with Speaker A and Speaker B. This would change the input options to 2 ohms mono, 8 ohms mono, 4 ohms stereo.

As you can see the switching is fairly complicated to reason through, and you could accidentally short the output of your amps if you forget to put the switch in the correct position.

I believe this is essentially and equivalent diagram from https://www.thegearpage.net/board/index.php?threads/stupid-questions-about-heads-and-cabs.1072198/


Here's what the jackplate the schematics were derived from looks like. FYI, 1960A uses four 16 ohms drivers connected in series parallel.

Jack Plate - Marshall, Switchable Stereo/Mono



Here's another diagram that is exactly what you asked for

Modify the left diagram by connecting an 8 ohm speaker in parallel with speaker 1 and speaker 2. This will give you exactly what you are asking for, 3 jacks, 4 ohms stereo, or 8 ohms mono.

 

If your existing cab is also 4 ohm, then you’ll need a 2 ohm capable head, which are not common.