The reason why I said 150 amp breaker for a main is while yes, it may be overkill in this current configuration, I am thinking down the road when you decide to upgrade, then you will have to upgrade your main power as well. While these circuit breakers are not a couple bucks a piece, you will want to buy one just once. I run a 200 amp breaker in my setup. Your main power protection circuit is designed to protect the vehicle in the event of a firewall short or short to ground, your distribution block should be a combination of splitter and fuses so your individual amps are protected. Your link is not working, but here is the style I am talking about
http://www.stinger-aamp.com/productDetails.aspx?ProductId=19&CategoryID=2 You don't have to use stinger, but one like this will work. 4 gauge will work fine, while each amp may be rated at 20 amperes each your amps for highs and mids will not be pulling a full 20 amperes, they might pull half the rated load, and if the amp running subs is pulling full load, your whole setup is only pulling 40 amperes...6 gauge cable is capable of sustaining 60 amperes continuous load, so 4 gauge will do just fine. Even if you run two sub amps, your still only going to pull 50 amperes total.
SVT