I have no idea what you mean about getting “a 4”. What units are you referring to…volts, decibels, milliamps?
To get more depth you can adjust the tone trim pots (high and low) to allow more signal to get through the optocouplers. You can try turning them both up the same number of turns until one of them maxes out….just so they keep the same ratio of high and lows.
Good luck!
-Brach