In my experience, unless you want to do all the midi programming its easiest to buy the controller that's designed for your software, i.e. the S4. It might cost you more than other options that will do the same thing but it will be the most intuitive and get the best support for controlling traktor.
There are other ways of performing but if you want to DJ thats a good start. You'll also need some headphones, and ideally some speakers. I think the S4 has a sound card in it so in that case you're good to go.
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