4 CDJs can be useful when playing loops. You can find parts from multiple different tracks and create these interesting live blends. Example - drums from one track, vocal from another and harmonic content on a third or fourth. These things usually have to be rehearsed though and work best with a mixer like the V10 or Model 1 In practice I tend to use 2 to mix tracks and a third for acapellas. Keeping in mind less is more and the listener’s ears can only keep up with so much