Dang, I was hoping for a more concrete answer. I want to build the 4pi because it's smaller and it would be awesome to have a 15 up front, but the 3pi has a -3db probably 20hz lower than the 4pi. Maybe I should just bite the bullet and do subs now? Originally I wanted Rythmik subs, but I'm not sure they're worth 3x the price of your pi3 sub.
Is it a dumb idea to build a subwoofer that is a stand for the 3pi or 4pi?
Which drivers are used in the posted frequency response curves? Does it change with different drivers, or are the similar enough that it's the same?