I think that you would be much happier with four Dayton or Stereo Integrity 18" drivers in ported cabs tuned to 16hz. I have heard the THT, and the Dayton HO18, and I much preferred the Dayton. I can not speak for the other horns, such as the Wrecker, (or what ever its called, lol).
Four of those 18's would be a truly world class setup, especially considering that you have a fairly smallish room.
Oh and one last thing, that is not how you model in winISD. You just model one driver/enclosure at a time, then use the laws of physics to estimate the SPL, ie....if one SI18 will do 110db at 20hz, then two SI18's will do 113db at 20hz, and so on. The way you did it, winISD will think you have an enclosure that contains 4 subs. Now granted, I don't use winISD, and u could be completely off base here!