Since dJay does not support 4 turntables, I switch between djay for iPad and djay for Mac. It would be awesome to be able to sync multi copies of djay together for beat matching. A cool bonus of this function would multiple DJs could sync their copies of djay and create some insane mixes.
I hear you. This is my key wish as well: to be able to run two iPads through a mixer and be able to beat match between the two pieces of hardware. Four decks with full app functionality? Solved.