
Because of how Traktor routes the audio, unfortunately you wouldn't be able to record the RMX without at least one additional soundcard (with multiple I/O's). The master output from the S4 would need to go through the RMX to the input of another audio interface, with that audio interface being merged with the audio interface of the S4 (either through ASIO4ALL on the PC or by creating an aggregate audio device on Mac), to record THAT input either within Traktor or another software, then sending the output signal to the 2nd audio interface for master output to your speakers.
It's not easy, but NI didn't make it easy. If you need further assistance, please let me know.