The AMP requires a 90A fuse.

Be sure you have enough alternator capacity, along with batteries, to
actually drive that amp. You may find you have neither enough battery
capacity, nor an alternator capable of keeping up with it. Factor in the
cost of a marinized alternator then.

I plan on wiring the 12V+ From the AMP (4 gauge) to the COMMON on the
switch. I have a 1' 4 gauge power cable with a 90A fues built in.

What you "plan" on wiring should be based on what the manufacturer of that
amp (which you haven't mentioned) recommends. It still doesn't sound like
you're on the right track.