4 400s would make for an incredible audio system. a 500 plays twice as loudly as a 400 ie. 106db vs. 109db anechoic but has the same extension and what looks like a 12db rolloff per octave rather than brickwall digital filters at 100hz in the 500/600. In some applications, especially with monitors that sloping rolloff can be helpful when blending one sub with speakers. On the other hand multiple subs with long rolloffs can pose a challenge for the mid-high bass output of the mains. The brickwall filters with their steep rolloffs are very useful in keeping sub sound and harmonics out of the higher frequencies which is the job of the speakers. That rolloff characteristic would more than anything distinguish the sonic qualities of the 400 compared to the other EP subs. The 400 has slightly less extension and much less output than the bigger subs but with four I would rule out output as a limiting factor in any room. Heck just a pair of them would make a big impact in any but the largest rooms.
From the perspective of smoothing FR no question four 400s would be just as effective as four 500s or 600s. Without actually testing these subs I'm hesitate to update that table. However I would venture that a system with 4 400s could overall make a better sonic presentation than a pair of EP600s because of their ability to flatten room nodes. The giveup would be in 3hz of extension favouring the 600s. Output would not necessarily be compromised because of coupling among the subs. Two subs potentially can increase output up to 6db depending on distance between them while 4 subs have the potential to add up to a further 12db.
Could be an interesting followup project...
