Adding a Potentiometer to control the volume of a sub-woofer
The problem what you will have is that the Amp is wanting an output impedance of 4-32 ohms (depending on the amp). This can do funny things the amps insides and may distort the sound.
The best location for controlling volume is at the pre-amp stage. 

If the 4 conductor wire is carrying the amplified signal, it not a good idea to run the low signal next to them. The amplified signal will disturb the low signal. If each of the 4 wires are shielded then you may get away with it.