AWG # 8
10 amps is the maximum it will carry.
Awg #6
The maximum amperage it can carry for power transmission is .92 amps.
40 amp
A #14 wire will do fine for grounding a 20 amp device. That is the size of the ground wire in a 2 conductor # 12 wire building cable.
si
Yes, they are the same size wire.
15 amps.
on a 4000 watt amp the best gauge wire woul be between 2 and 0
You will need a 40 amp breaker using AWG # 8 wire running it a distance of no more than 50 feet.
You use the correct size breaker depending on the size wire in the circuit. If the circuit is wired with AWG #12 wire use a 20 amp breaker. If it is wired with AWG #14 wire then use a 15 amp breaker.