You would be better off making your own. Go to your local car audio shop and tell them you need a piece of 4 awg and some ring terminals and you can make it what ever length you want.
If you are going through the trouble of upgrading power/ground wire I would really recommend replacing them all as its not a bid deal and can help to reduce problems later.
There are 3 grounds - battery to chassis - Battery to engine - Chassis to block