I would go with #4 because if I have to do a job like that, I only want to do it once. #4 wire is a pain in the butt to work with. It is like trying to pull a 2x4 thru the maze of crap that is typically in the underside of our boat floors. That said, you will get much less power loss thru the large wire. The last 3 boats I have rigged got #4 wire from the TM batteries to the TM because I eventually went to what could be considered over-kill on my trolling motor sizes. I would rather have it and never need it than wish I had it and didn't.