you will need an expression pedal/vol pedal with the cable attached directly to the pot in the exp pedal/vol pedal itself. The cable to connect need to be stereo cable and the plug is usually stereo plug. The reason being one terminal of the stereo plug is wired as signal send to pot(in exp pedal) and the other terminal for signal return after the value changed, according to the pot rotation in the exp pedal
the ernie ball one will have to rewire a bit of the inside to use for exp control. Take out the output jack, rewire a stereo jack to the pot in it and use a stereo cable(with stereo plug) to connect to the fm4
the easier way is just get an expression pedal to use with the fm4. I have been using proel exp pedal with various line 6 modellers. Its cheap(compare to other brand of exp pedal) and workable