4 DECKS SETUP? - NUMARK MIXTRACK PRO2 + DJM900NXS + CDJ2000NXS

Hi,
I am quite new when it comes to setup.
I have upgraded my equipment to Pioneer DJM+CDJ and I was wondering if there is a way to keep using Numark Mixtrack pro 2 as additional decks with the external mixer.
I have connected the Mixtrack Pro 2 to the DJM900NXS with a RCA cable but it works just on deck 1 & 2 even when is connected to the decks 3 or 4

Does anyone know how to set it up to have 4 decks?