most new style rca’s are good enough that you wont get that alternator whine or interference. you can run all the wires together if you get a true quality set of rca’s. dont even hook up the amp until you can run the rca’s properly as was previously mentioned. also… why would you need to run two 4ga. wires?.. run 4 gauge or 0/1 gauge to a distribution block and split it there. SOOO much cheaper and cleaner. depending on the wattage and amperage you may want to look into a 1-3 farad capacitor for the sub amp only. run the power wire (fuse inline) from battery to the distro block, split it into 2 4gauge wires, 1 going to the interior amp the other going into the capacitor, then from the capacitor to the sub amp… helps with voltage drop…i was running 3 amps putting out over 2000 watts at one point with around a 140 amp draw on my stock electrical system and if i wasnt driving or revving the car and i had the music turned up the car would actually shut off on me from the bass. if you want help id be glad to lend a hand. (assuming you have someplace semi-warm to do this)… stereo shit in the cold like this is KILLER on ur fingers.