Considering the prices of 4-channel amplifiers, I really don't see a reason to go through this kludge. If you want to save space, get a space efficient Class D 4-channel amplifier. Such as PPI P900.4. The dimensions are something like 10 by 7 inches, and it has a massive amount of power for either speakers or subwoofers (unless this is an SPL setup). If you want to amplify all speaker in the car, consider the 5-channel version, P900.5 or Soundstream TN5.950D.