Difficulty choosing the right DJ Controller

They are all excellent controllers, depending on what software you would like to stick with and your mixing style. The S4 has extremely tight integration and midi communication with Traktor. The 4Trak looks pretty legit and can be an independent mixer which is a handy feature + the effects section looks good. I haven’t used one however. The NS6 is solid and a good contender if you use Itch.

Choose your software first, then pick your controller.

Sounds like you’re swaying towards Traktor judging by the controllers you picked ie solid effects sections. Best software in my opinion (easily has best effects and mapping creativity options) but do what works for you.