The gauge also depends on the length of the setup. If you have your battery in the back of the car then your gauge does not need to be as thick and if you drive a long suburban with a 20 ft power cord it would need to be thicker all things being equal.
ditoo... 4 gauge from batter to distribution block.. then 8 ga power cables from distribution block to amps... then the amps would require the same gauge wire to be grounded with(8ga.)..
(1) 4gauge wire(length from battery to distribution blocks(close to amps)
(2) 8gauge power cables (length from dist block to amp)