The four channels are also quite similar to the EQ section of a DJ mixer which generally controls three frequency ranges (low, mid and high). EQ-ing is heavily used to mix songs together, so by sticking to a similar interface, DJ's can easily pick up this format.

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.