The Main fuse at the battery should be the sum of you adding the 2 Amp's ratings together.
I'd say the 300 should be a 40A.
The 4-Way should be a 50A.
So go with a 80A or 100A.
Only think is that AGU fuses only go to 80A.
I'm running an 80A main for a 1000 WATT RMS mono block and then a second 80A between the Cap and the Amp.
On the block you can go with the fuses that match the Amps. Or just 2 50A fuses.
