I want to run 4 gauge from battery down driver side to back of the backseat on driver side where my amps will be. Approximately how much length would I need?
buy an amp kit with everything in it. Normally you'll need 17-20 feet. To get to that rear quarter panel you'll prob need more, or get a distribution block which allows you to properly extend the wire with some scrap you can find or buy at a shop for cheap.
You could also take a tap measure and get pretty close, then add a foot or so just for safety.
figure about half the length of that string of question marks.
tarheel is right. I'd buy an amp kit to fit your wattage needs. And if the power cable included isn't long enough, you can install a dist block or an inline fuse which will allow you to run more cable. You can buy cable by the foot at any car audio place.