If there is a welding supply store in your area, check into getting a ground cable made up from #0 or larger welding cable. The #4 cable you can get from the FLAPS is going to be a bit under-capacity for good operation with a 6V system. Most welding supply places will have a way of doing a very low-resistance crimp connection of terminal lugs onto the cable, and welding cable is made from hundreds of strands of hair-fine wire that will have lower resistance than a similar-diameter piece of battery cable. For the absolute lowest-resistance terminal connections, use a Propane torch and lots of rosin-core solder to attach the terminal lugs. That's how we built battery cables for the race cars with 11:1 compression or higher.
Jerry
