In a modular synth, this can be done with three crossfader modules. One connect to A and B, and the other to C and D inputs. The third connects to the outputs of the other two. The same CV (the x axis on a joystick for example) is applied to the first two crossfaders and the second CV (the Y axis) is applied to the third crossfader. The final output is taken from the third crossfader.
To do quadraphonic panning, you just do it in reverse but use panners and not crossfaders.