A 4ga wire transferring 100amps will drop 5% of it's current over a 12ft run, or about .7 volts. 4ga can sustain 135 amps over short runs (under 20ft) with no issues. If you want to know why you're blowing fuses, don't look to the wire-- voltage drop doesn't blow fuses. You won't need to step to a huge wire gauge until you're good and well past 2000 watts RMS.

I'm running a Fosgate 400a4 and 800a2 off of a single 4ga from battery back, and have never had fuse or voltage dip problems (135a alternator). There's no need to include a main power line with more capacity than your entire charging system.