Easiest would be to just record the master out from you Numark.
By the look of it the N4 has an option of sending one stereo channel of sound back to the software. You need to make sure the switch on the back is set to USB in Rec (not timecode). Then you need to set the correct input channel in Traktor.
Under Preferences:
Tab Audio setup – under “phone/’line” – Input channel. Here it should give you the option of picking the N4 input channel.
Tab Input routing – under input Deck C – choose both L + R channel
That should do the trick. I am a bit rusty, so anyone that knows more and/or better, don’t hesitate to correct me 😀