www.bcae1.com. learn it. live it.
an opus is 150 watts, then it runs about 10-15 feet from your battery. 8 gauge is fine, 10 could also do it, but anything thinner might pose problems. it's true you can't use your stock radio's power wire, but you most definitely won't need 4 gauge wiring. to add to that, 4gauge is thick. really hard to get through your stock firewall, most likely you might have to drill a new hole for it.
just my .02$