What you have to ask yourself is:
What do you expect from CDJs and a mixer (if you want to use Traktor, like you say) that an S4, 4Trak, DDJ-SX cannot do?
Traktor is a very good software, but there is no point in using it with CDJs if you want to go fully digital anyways.
The only thing I could see is, if you do not want to be dependend on a laptop and want to spin older CDs or songs from a USB drive also. But you pay for this with a much more expensive setup and hugely more heavy to transport, as you would need flightcases to protect the CDJs. So we are talking 4000 bucks or more as for the same flexibility you would need the CDJ 2000 Nexus and a DJM 900 or better at least. I see not much advantage in that.
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