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. <a href="https:&#x2F;&#x2F;pro.beatport.com&#x2F;stems&#x2F;about" rel="nofollow">https:&#x2F;&#x2F;pro.beatport.com&#x2F;stems&#x2F;about</a><p>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.