Re: How many Iches²
I would try to get at least 100" for (4) 12's.
If you do not have room, you can look at flared 4" aero ports. The large flares will allow for a lot less port area. You could use 4 of them 14" long for 35 Hz tuning, or 5 of them 17" long for 35 Hz tuning with your setup.
edit - I read fast and thought you were using DD subs, but you only say you read the DD site. The required port area will also depend on what subs you are using and how much power you have. A cheap sub with not much power can get away with less port area. But even if that is the case, you might as well build it right for upgrades in the future
2000 Suburban - (4) BTL 15's and (4) IA 40.1's = 157.7 dB at 37 Hz.
1992 Astro Van - (6) BTL 15's and (6) IA 40.1's = 159.7 dB at 43 Hz.
no system right now. . .
Next system: (2) Massive Nx4 amps, (1) Massive N4 amp, (1) Audiocontrol EQL and 2XS
(12) AA Carbon Fiber 7" mids, (8) Morel MDT-12 tweeters, (1) 15" UFO BTL
REFS: iceteebone,jntar,djman37,TeamPSI,James Bang,96MaxGLE,photocrazy8,xclusiv3,DBDesignsElite, Amalan