To quantify the differences, first know that the lower -3dB point is virtually the same on both speakers, at 50Hz. That, in itself, is significant because the Theater three π is much smaller than the Theater four π. The four π is more than twice as large as the three π, so that may be an important consideration for many people.

But the the four π has more rising response through the midrange, and so is louder but also depends more on room placement to bring up the depest bass. The three π has a response curve that looks almost like a sealed box, with a slow but consistent rolloff to its -10dB point at 27Hz. The four π has a curve that's more like an EBS alignment, which means there is a shelved response that's good down to about 20Hz. That's about a half octave lower than the three π goes, but again, with more of a rising response profile through the midrange.

Size and efficiency are important considerations in favor of the three π speakers. Since you're planning to match this bass system with one π's - effectively making a three-way system with the one π speakers forming the midrange and tweeter subsystems - you might like the three π cabinet because it matches in efficiency and has a very attractive small "footprint."

One final thing - If you plan to run a tube amp, the four π speaker will be perfectly fine, but the three π speaker might work best with push-pull tube amps or solid state. If you plan to run a SET amp, it probably should be pretty powerful with good damping ability if you plan to drive three π's. It isn't so much the power I'm looking for, because these are very efficient speakers. But the three π woofers work best with an amp with good damping ability.

So there are valid reasons to go with each speaker; There are strengths and weaknesses with each one. Let us know which you choose, and how it works out!