Do NOT use the deck to power the rear speakers.
Some of the most important things in sound quality for car audio is clean, consistent power and predictable output from your components.

However, you can still use the 4-channel amp to do the entire system. Take the front 2 channels of the amp and push all 4 front & rear speakers, equalling a 2-ohm stereo load. This is ok since music is recorded in stereo (left and right), not front and rear. You will lose Fading capabilites, but who uses that anyways?

Then take the rear channels of the amp and sum them for a increased mono ouput for the sub. You now have a fully amplified system that will sound much better than if you ever use the 12-15 watts of useless power from the CD player.