well 2x WO, better yet, 2x WO32 built into a permanent installation would be "sufficient" for that space if you took advantage of the built in part.  See, if you load a WO, or WO32, in to a corner, you can make it even more like a horn, increasing both the SPL, and the low extension of the enclosures.  There are others here with more experience on horns, and the WO "horn" who can hopefully fill in the blanks for you.

Four would be ridiculous for that space, especially if you stacked tham all in one corner, likely not the kind of experience you seek I would imagine.  

umm, a single Imperial SO would also likely cover most of your needs in the subwoofer dept, make the mains Conventional Imperials paired to some nice DIY wooden horns for the highs and you would have a handsome system indeed.  Likely the 25 sheets of ply for this would cost more than you are willing to spend all by themselves, but the imperials seem to be very tolerant of most any driver.  The SO needs a driver that hits 20Hz comfortably in a 5 cube box, which can sometimes be "tricky", but in either case, a few dollars saved on drivers will likely go unnoticed given the intended application.

There are several threads around here about using either of the subs i mentioned, and the imperials in PA applications.

hope that sheds some light on your question....