yes 4ga is only rated for up to 125A and that 1200 watt amp only needs about a 115-120A fuse itself 0ga can take up to 325A, and 00ga is good to 400A. These are peak ratings though and will decrease based on the distance you run the cable.
lol i have a 1600 watt amp mono to 2, 12" audiobahn .... we both are on the same boat.. sort of.. you need a high output alternator ... these batterys that they make that are badass.. and you dont really need a power cap but like i dunno.. with the fuse i would say ask a professional you want the exact voltage
