Power Wire size Vs. Ground Wire size

I have just purchased a 4 channel amp (Alpine MRP-F450) to run my interior speakers.

I currently have 4 ga. ran in my car from battery to trunk, which is way more than my current 400watt monoblock requires on its own. I think that the total fuse rating between the two amps is somewhere between 90 and 110 amps, I can't remember off of the top of my head.

My new alpine uses 2 x 30A fuses on its own but it will only be running @ 4 ohm on all channels, none of them bridged. Because of this I believe my 4 ga from battery to trunk is still ok to use.

My question lies with the wiring once in the trunk. I obviously have to go and pick up a distribution block and based on the owner's manuals for both amps each amp only requires 8 ga. Is it ok to take the run of 4 ga. back to the trunk, split it to 2 8 ga. power leads, and then ground each amp with 4 ga. grounds?

The reason why I ask is that I have enough scrap 4 and 8 ga. around to do the install but only in this way. Would I be better off just going out and getting some more 8 ga. so that power and ground on each amp are the same size?

Also, must the 2 amps be grounded at the same point? I think the answer is yes, but it would be a lot easier if it were no.

Thanks

That's what I figured,  that's why I said I'd use the 4 ga. as grounds. I'm hoping this might help too because my grounds might be a bit longer than 3 ft. especially if they must ground at the same point.

Speaking of which is this truly necessary? By my reading I've heard that ground loops may develope when multiple amps have the same power source but different grounds. Is this myth or fact?

I was hoping to install my 2 amps on the 2 opposite sides of the trunk.