In my experience, you CAN use the RMX-1000 w/ the Kontrol s4; however, it's not ideal and recording the FX is not possible, at all.

Basically, if you are using the Kontrol s4, you are using a unit that does not support SEND/RETURN. This will hold true if you are using any system without a SEND/RETURN. In order for you to experience what the RMX-1000 has to offer, you will need to connect the master output of your mixer into the input of the RMX-1000 and the output of the RMX-1000 to the stereo amplifier/speaker. With this setup you will only be able to hear the effects applied out of the stereo amplifier directly. No more, no less.

Hope this helps some. If you are using the Kontrol s4, you should stick to the FX in Traktor. MANY, if not all of what the RMX-1000 can do, you can natively do in Traktor.

With that said, I'm willing to sell my RMX-1000 for $650. I used it for 5 minutes to verify the above comments are accurate. It's practically brand new and $150 less than you'd get it in stores.