Multiplexing 4 DC motors with arduino
Why do you need 4 inputs per fader? I presume that one is for sensing position, and one is for the actual gain of the fader (which is supposed to be analog) -- the Arduino only needs to read one of them. If that's a voltage divider between 5V and GND, you only get one analog input per fader for position.

If touch is a digital signal, or can be determined with a comparator, you can then use a digital input for that. Or, as you said, ignore touch.

Finally, you have 6 PWM outputs, but only need 4, and then another output each for direction, assuming you have a reasonable motor controller with enable/direction inputs.

So on an Arduino UNO:


position sensor: A0 through A3 (save A4 and A5 for I2C)
motor PWM (speed): four of the PWM digital outputs
motor direction: four of the non-PWM digital outputs
touch sense (if needed/implemented): four non-PWM digital inputs


Total, 12 digital pins, 4 analog pins. Leaves the serial port (D0/D1) free for programming, and I2C (A4/A5) free for I2C.