Active speakers you wont need a mixer, since its already built into the N4. You just gotta give the speakers power by plugging them into an outlet.
In Traktor, you go up to Setting (its the button on the top that looks like a gear). You go to audio setup and then under Audio Device you select your mixer.
If your mixer isnt there, than you may need to download the TSI file for your N4. You can find that http://www.numark.com/product/n4. Its on the side bar on the right, the first file under the downloads. Once its downloaded, there are instructions in the zip file on how to install it.
Happy Spinning!