When using a pair of four π speakers, I think it makes sense to stack them with midwoofers outboard above and below, i.e. MTTM. That's better than MTMT, which would leave too much distance between tweeters.For larger installs, a modular line array approach would probably be better, with a square mouth tweeter on one side and a woofer on the other, both being approximately the same size. When vertically stacked, they form a line of closely spaced midwoofers and a line of closely spaced tweeters. That's basically a four π on its side, using a different tweeter appropriate to the orientation and application.