What is the best way to route the 4 gauge power wire through the firewall, and the rca cable from behind the head unit? 2003 protege sedan DX

 

I don't know about the routing, but please make sure you add another ground wire from the negative terminal of battery to the body of the car and put a fuse or breaker on the positive 4 gauge wire you are installing just before the battery.

 

I know about the fuse on the amp power wire, but why add another ground wire from the battery?

 

the stock ground is meant for stuff on the car, not any more stuff especially hi powered amps. The stock ground wire isn't big enough for all the stuff with your amp.
You install a ground wire near your amp, but people forget to add another one by battery. I know a guy who didn't do this (on another vehicle) and his ground wire fried.
You have to properly complete the circuit!

"the primary performance limitation in most amplifier installations is in the current delivery — either a weak ground or insufficient wire gauge. Installing too small of a wire gauge results in poor performance, potentially shorter service life of connected components (your amplifier and speakers), and a potential safety hazard."