I am running 0 in my car because I will be running 3 amps. :stirthepot::tup:

As a general rule, 4 Ga. is good for up to 100 amps (at 20' length)
anything above 100 amps, go to 2 Ga or 0 Ga.

You are correct that it depends on what he is running but I've competed in my younger days on a 4 gauge 3 amp 1200 system drawing 100 amps without any issues whatsoever. 4 gauge at 100 amps will be good up to 12 feet. In a Z, 4 gauge will be good for a 120 amp 1200-1400 watt set up. It is a small cabin with cables not running that far. In addition, all things being equal i.e. same h/u, speakers, amplification, RCA cabling, same length running power cables, etc. the human ear will not be able to distinguish any difference in SQ and SPL by using 0 vs 4 gauge cable. The current needed to drive these amps can easily be supplied by 4 gauge cabling in most cases especially in a Z. If you have room to put a system that draws major current in a small cabin like the z use 0 but if you have a modest 1200-1500 watt system in a small car use 4 g and spend the money on better h/u, amplification, sound processing or RCA's or speakers where it really counts.
If you already have the 0 g sitting around then use it by all means. Enjoy your music and protect your ears. I can say this with experience :)