questions about battery disconnect wiring

Crackedback's wiring method has the advantage of leaving the big feed to the starter dead except for starting.
67Dart's approach has the advantage of a shorter path for alternator power.
You could combine them.

The way I drew it above, only a 16 ga fusible link is needed. That protects the 12 ga wires. If the feed to the starter grounds, or even the one up to the starter relay, the battery will discharge through them until the connection is broken.

If something needs a 4 ga wire, then maybe it can be protected with a maxi fuse.
More likely what you can reasonably protect is the smaller wires after the 4 ga wire.
You may want to do both.