The slider is a potentiometer, a variable resistor. Although I forget what it'* value is (1, 10, 100k ohms), you might be able to run your pre-outs directly though it. It'* made to handle very small amounts of power, maybe even your 4 volts.
The problem: It responds flatly, not on a curve, so it may not do what you want it to. Variable resistors are also selected specifically for an application (since there are many types and values), not vice versa. It won't probably have the range you want in volumes, and probably won't start or end at the right volume, either. If it does work, it might turn out to be very precise, too precise to use.
The point is that someone should try it, and if it doesn't work like everyone wants, let us know. I can get together with some people and design some circuitry to complement it so it does what everyone wants it to do, since it is becoming a pretty popular request.
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU