Im a bit puzzled. I have a power wire running from my battery to my existing amp in my car, but I'm not sure what gauge the wire is. The wire casing hasn't got it labled on it, only that its 10mm2. I'm not sure but I think it's an 8 gauge wire. If so I am thinking of replacing it with a 4 gauge cose im gonna hook up 2 amps. What 'mm2' is a 4 gauge wire?