power wire and distribution block

They're needed, so you should use them.  Although you could cut some corners and install the fuse value rating for 4 gauge in the main wire, you would be allowing the possibility for catastrophic failure later down the road. 

Scenario:  two years from now, you or someone decides to add another amplifier into the circuit.  The initial installation was far in the past, so some of the details are cloudy.  You see that 0 gauge wire is provided, so the device is installed.  The fuse value on the 0 ga. wire is looked at and considered too small for the application, so it is upped in value to 300 amps.  The two existing 4 gauge wires feeding out to the two year old amps are buried in the installation and overlooked (is it unfathomable to overlook this part of the installation that has never failed, as they have nothing to do with the newly added device?)  These wires now are not protected from getting hot enough to start a fire.

In an automobile, chances of catastrophic failure would be greater than on your boat because the entire chassis is ground.  It is easier, in that case, to envision how suddenly the battery reserves could get dumped through the circuit all at once.  But you should provide the same basic safety to your boat as much as you would to your car.  It is the standard for circuit protection to fuse to wire value.

The long and short of it is, you may not be standing at the ready with a fire extinguisher if a sudden catastrophe were to happen to your boat.  Don't cut corners.  Protect each wire at its rating and you will have set up the wiring correctly.  "2-6-8" the job and you will bear the burden of remembering what shortcuts you took from now on.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

The purpose of a fuse is to protect the wire and the vehicle.  If you pull more current through a wire then it is rated for then the wire will heat up and possibly catch on fire.

The "problem" you have here is you have two different sizes of wire.  If you fuse your 1/0 at 300A (which seems WAY too high for this install) and you don't fuse your 4ga wire then you have the possibility of running 300A through a wire that is only rated to handle ~150A.  This can cause the 4ga to heat up and catch on fire (although this is very unlikely in your install because the 4ga wire in your install is so short).

So, you can do two differnet things - fuse the entire system at the level of the 4ga wire (150A or so) or fuse both legs (1/0 and 4ga) seperately.  If you have the extra fuse holders and you don't mind spending the money I would highly recomend the fuse holders in the back.  If you're trying to save money you can technically be safe and not use them IF your front fuse is 150A or smaller. 

It would be really interesting to see what kind of loads both amps are going to draw from the battery under full load as that is the most important information to have when properly sizing the system fuses.