If you're using Audyssey (or similar DRC) to smooth FR in the bass, then a pair of 18s probably makes more sense. I believe that you'll have pretty similar clean SPL capability all the way down well below 20hz from either set-up, and 2 giant boys are probably cheaper and easier to place than 4 big boys. As you note, it's probably overkill in either case (unless you got yourself one very big HT room), so there's little chance of going wrong, either way.
If you're not digitally correcting for the room, then 4 subs will probably get you smoother response. In that case, you might want to place the "top" sub on a support that's placed closer to the ceiling rather than stacked on the "bottom" sub. I'd think that this placement would take the ceiling reflections out of play (to a degree) and this should get smoother FR in the bass.
Marty
If you're not digitally correcting for the room, then 4 subs will probably get you smoother response. In that case, you might want to place the "top" sub on a support that's placed closer to the ceiling rather than stacked on the "bottom" sub. I'd think that this placement would take the ceiling reflections out of play (to a degree) and this should get smoother FR in the bass.
Marty