Underhood electrical wiring

Ok. After a half hour, I have given up. Where do the larger wires (looks like 4ga) from the positive and negative battery terminals go?
Two wires come off the negative terminal...one to chassis ground right next to the battery, the larger one off to maybe the alternator case?
Two wires come off the positive terminal, one to the underhood fuse box, the other...no idea whatsoever. I lose it right below the alternator.
If you're wondering why I am asking, I am replacing the battery terminals with aftermarket, and the easiest thing to do (or so I thought) is to replace the wires that attach to the terminals as well. Call it the Big 2 upgrade.
Can anyone help?

Matt

 

Positive - Fuse box and Starter Solenoid.
Negative - Body/Chassis and Engine block.