[V]

have you confirmed that much voltage drop at the battery currently? For some reason i can't believe there is that much loss. that would imply the taillights on any car will have a substantial drop as well due to being a thinner wire, therefore more resistance. Yet a voltmeter shows only like .2-.3 volt drop, not 1.5+(edit but thats less current..so nevermind lol). I could be wrong though, but i say just use a volt meter and confirm what you're charging voltage at the rear battery is with the 4 awg. If you do see under 13.2v, then id suggest going to a bigger gauge wire.