The bugaboo in 6 volt systems is something called "voltage drop". Think of a piece of wire as a water hose, with the voltage being the pressure that forces the water through it, and the current being the gallons per hour of water flow. A big hose with the same pressure against it will flow more water than a small one, right? By the same logic, losing a little pressure on the big hose will have less of an effect on total flow than if the small hose lost the same amount of pressure.
Current flow (amperes) is what makes a starter do its job, and whatever voltage is consumed pushing current through a too-small wire isn't there to push the current through the starter windings. Bigger is always better where wire size is concerned, and 4 gauge is just TOO SMALL to work with a 6 volt system if it's the only path for the current to get from the starter housing back to the battery. That's why people opt for a ground cable attached directly to the engine/transmission castings, rather than do it the factory way with two cables, one grounded to the frame, and another one frame to engine. That's twice as many connections to get loose or dirty, and more places to lose voltage. Run the 0 gauge battery ground to the engine, and let the little 4 gauge engine-to-frame jumper be the ground for the headlights, horn, heater blower, and other body-mounted accessories.
Jerry
