Having built both 3Pi and 4Pi theaters I think what you have planned could work well. I do think I should mention that the 3Pi does not come off like the 4's little brother. The 3 is smaller, like Bruce Lee! If you feel the 4's don't need a sub, you may find the same with the 3's, but it is nice to build speakers that will work without a sub as the times and systems change. The bottom two octaves of both systems are very close and equally deep. If anything it is the midbass and lower mids where the 4Pi is a bit stronger. In a smaller room the 3 could equal or better the 4 in a big room. I wouldn't care which I had for 95% of my listening.

Thomas