[Subwoofer design and calc for Mac]

To use this program I use VMware fusion and download windows. This program allows you to run Mac OSX and windows on the same computer with out have to have one or the other.

[DC level 4 or Audioque HDC3]

DC level 4 or Audioque HDC3 which will be louder and which one would you go with.

[American bass xfl 12s]

American bass under rates they can take a lot of power. Good choice for your system.

[AMP for dc lvl 4 12]

The AQ 1200 would be your best fit then and Im pretty sure it is 1 ohm stable so that would be perfect for your system

[4 obsidian audio 15s]

I think the subs would be able to take the power it's just your budget can't be low. Your upgrade on the mids and high should maybe wait if you get the amp because your going to be spending money on another battery which will jack up your budget if you are going to get another amp.

[4000 watt amps?]

You should consider raising your budget because by the time you are done you will just be spend 500 on a battery or two. Possible a larger alternator with more amps like a 200 amp alternator.