To be honest, a 4 gauge will work marginally. If the bat is ever weak for any reason, you are likely to have a no start condition. It sounds like your problem is one of 2 things.
1) battery is not charging. Use volt meter and verify 14 volts at 1500rpm.
2) bad connection somewhere. Clean battery posts, ground (remove paint) and recheck all connections.
Other suggestions are to double up on your 4 gauge wire. That will work and you wont waste the 4 gauge you have. Run a ground from the body/frame to the motor. Scrape the paint till you get shiny metal then put the cable on. They you can spray over the whole thing to prevent rust and enhance appearance.