Four is actually ideal. Floyd Toole, who I guess is some hotshot in acoustics hehe, at least I found his papers interesting and got real results from reading them, found that two got you 90% of the way and four got you about as good as it could get. Most cases, the four subwoofers were placed in the center of each wall.
Over four subwoofers, you want to move to 8, which you'd stack to get an extra +3dB from arraying. But that would only be worthwhile if your room was so big it needed the output, it wouldn't help the response.
Fact of the matter is, most high-end home theaters have such weak mains compared to their subwoofers that output isn't the weak link. Hence, why I tell people to look at stuff like JTR's, Danleys and Seatons, for home theater because they can actually provide that 'punch' people want but haven't figured out comes from about 500Hz hehe, which some little tower aint gunna handle. Instead, they put giant subwoofer(s), run them way too hot over their mains, and the result is muddy.
When people ask me how to get more 'punch' from their bass, generally I tell them to turn their subwoofer down and their receiver up. They are amazed. Want some extra growl, go +3dB on the sub...which is borderline. Want some punch, get some massive mains that can do the lower midrange crunch that makes explosions feel like gunshots to your chest.