Seven Pi cornerhorns are the only speakers I know of that I would consider to be a "step up" from four Pi's.Let me make a suggestion: Why not build your four Pi's and listen to them for a while before considering what you may or may not want as an "upgrade." The four Pi speaker is a very capable speaker, and at risk of sounding immodest I have to admit I know of no better loudspeaker of its size, anywhere.