I was wondering the same, and found that there are already available stems on Beatport, where they classify the 4 tracks as: Drum, Bass, Vocals, and Theme. http

I was wondering the same, and found that there are already available stems on Beatport, where they classify the 4 tracks as: Drum, Bass, Vocals, and Theme. https://pro.beatport.com/stems/about https://pro.beatport.com/stems/about I suppose DJ equipment manufacturers like Pioneer will step up and add volume controller, and mute and solo buttons for each track on CDJ units. So the fixed number of 4 tracks makes the hardware easier to design.