4gauge is fine for the amp wiring. I'm not really sure how to explain how to upgrade the stock wiring, so i'll let this other person's website do the explaining.

note: i didn't write this page, barely even looked at it actually, just searched the "big 3 audio upgrade" as it's usually referred to as the big 3

http://www.isyougeekedup.com/the-big...iring-upgrade/