Far as I know it's just size of wire, smaller the gauge the thicker the wire. which means the 4 is going to be thicker than the 8.
The ampacity or amp rating of all wire is rated by the size of the wire. NM (non-metallic sheathing) wire is no different. In household wiring 14 gauge wire must go on a 15 amp breaker/ 12 gauge goes on a 20 A and 10 gauge goes on a 30 Amp.
on a 4000 watt amp the best gauge wire woul be between 2 and 0
You'll need RCA style cables to connect from your headunit to the amp. You'll then need Speaker Wire to run from the outputs of the Amp to the speaker terminals. You'll also need a heavy gauge power wire. A wiring kit such as the StreetWires Power Station PSK04Ri 4-gauge amplifier wiring kit.
i would say a 4 gauge kit would be more then enough for it 8 gauge min...
An amp is the actual amp while an amp chord connects the instrument to the amp.
1 amp
NEC states that all GFCI's require a 20 amp circuit, the wire size for a 20 amp circuit is 12 gauge.
10 amps
Not much. Keyboard amp will have a better frequency range.
at least 4ga wire but it would be best to get 4awg or 0awg. 4 gauge is smaller than 4 American gauge wire
Yes, if the existing wiring you are adding to is 14 gauge. If it is 12 guage then you must use 12 gauge. If you are running wire from the service panel it is fine as long as you install a 15 amp breaker.
10 AWG.