If the amps have internal fusing then just fuse for the wire size, generally 4 gauge is 150A, 1/0 is 300A.
If the amps DO NOT have internal fusing then you need to fuse for the amplifier size. If you don't have the manuals, try to find them on google, they will tell you exactly what size fuse you should use.
If the amps have internal fusing and you just want to be extra careful (not really necessary) you can still fuse for the amplifier size. This will probably only affect your smaller amp seeing as your 1600w amp will probably need the max size (150A).