I say it depends. How large is your room? Do you listen alone or with other people, and if so how far appart are they? Or do you move around while you listen?
I believe multiple subs have the advantage of potentially higher SPL (particularly valuable in large rooms), and of smoothing the bass response across various points in the room (see Earl Geddes multiple sub approach).
I run a pair of stereo subs and I'm happy with them. I might consider adding more down the road, but have no burning need now. Bigger fish to catch elsewhere.
I did try the Geddes approach and worked well for me, but stereo subs works better for me now. I listen mostly alone and stay sitting in one place while doing so. If going down the Geddes route, you don't need all subs to be the same. Moreover, your 3rd and 4th sub could easily be smaller and you would loose nothing.
