I decided to go with a larger cab. I decreased the size a bit to make it fit better on a 4 x 8. 33 x 22 x 16.
I did a quick search to find the baffle spacing between the tweet and the woof for the 4Pi. I found a reference to 13.25" but I think that was the old Theater 4 Pi.
Judging from the picture of the 4Pi it looks closer to 12".