Will This be ok? Fuse & Wire?

You will only get to the amplifier's fuse rating if -

(and we are going to give you the benefit of the doubt that you have gains set correctly)

1.   the amps are outputting their highest rated wattage (lowest ohm load), and

2.  when, along with the rule of #1, you are playing the system at full output

When both those two rules don't apply, the current transfered through the power wire will be much less than the total amp fuse ratings.  So to answer your questions in order:

1.  You will never get to 120 amps of current in the wire.  It is fused with an 80 amp fuse at the battery.  You can have a vehicle load test done on the alternator (electrical system) at a local auto shop to find out how much available capacity it has.

2.  No, 4 ga. is sufficient.  See https://www.the12volt.com/info/recwirsz.asp

3.  Depends on how many watts the amps are putting out.  If your running the fronts at 4 ohms, and the sub at 2 ohms, you'll likely draw only half the total fuse ratings.  But if both amps are loaded to achieve max wattage, you will need to allow for 120 amps.

If you blow that 80 amp fuse, the amps are pulling more current than 80 amps.  If the volume was acceptable, and you are not setting up for competition,  then you can put another 80 amp fuse in the wire and keep the volume a little lower.  But you can see on the chart that a 4 ga. wire may be fused up to 120 amps if you wish.

Oh, and you will encounter a very hot wire if too many amps are flowing through it, for its size.  That's why the fuses.

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

Thanks so much> I think I'm good to go on switching out the coustic amp for the hifonics. The coustic is 40x4rms 4ohms and I'll be stepping up to 85x4rms 4ohms on the hifonics and then I have the jbl bp600.1 which I think is 300rms and is at 4ohms. According to the chart you referred me to everything looks fine with what I have. So do you think I'm good to make the switch without changing any wiring?

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