However should I use a fuse for the 4AWG power wire from the battery terminal to the fuse block? if so what rating?

It is only the most important fuse in your entire car, and the one most likely to be actually needed to prevent your car from catching fire.

The three smaller fuses on the power distribution block are mostly there to protect your amplifiers. The big fuse (I recommend 80 amp) that you need to put on the 4-gage wire running from your battery to the distribution block is there to protect your car.

That wire is the most likely one to be compromised, running through the firewall as it does and routed through improbable places to get to your distribution block. And even though that 4-gage wire can carry an impressive amount of current, your battery can deliver even more. If that wire were to short against your firewall, your battery can put out enough current to literally catch it (and anything else it is touching or even near) on fire in about five seconds or less. An 80-amp fuse in a short circuit will blow so fast that that wire won't even become warm to the touch, yet it will carry the 40 amp load that your amplifiers can draw forever.

You are correct that each device powered by the distribution block must have its own correctly sized fuse; but the distribution block itself must be fused as well, as close to the battery as possible.

tanstaafl.