battery in the trunk problems!!

Do a search for this there are lots of detailed instructions. Booster cable wire works well.

4 Ga may be OK for a VW, but way too small for anything that would benefit with a battery relocation. The smallest I would use with even a small block with low compression is 2 Ga. 1 Ga or O Ga is preferred, with 00 being for the big block with 12-1 or more.

I used 1/0 welding cable for my power wire, 2/0 welding cable for my battery ground (it's THICK wire) and the stock block ground cable which i think is 4ga.

Cleaned all the grounds REALLY well, and i had 0 issues.

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.

Put in another run of 4g wire. It will have the same resistance as 1/0 guage and it will be a hell of alot easier to work with and hide. Heck its cheaper too - 4g is about $2/ft and 1/0g is about $6/ft. Just use car audio power cable - it has more strands and less resistance than welding cable. As far as grounds go, get a big fatty ground wire and find a place that goes directly into the frame - I used the bolts for the rear seat belts. Upgrade your grounds under your hood & you may want to even ground the alternator directly to the chassis. And just keep in mind, the fewer the ground points you have - the better. If there is a place where you can ground everything DO IT! Hope this helps.