if you apply the external mixer hack (it’s a simple edit of the config xml, and can be found on the traktorbible website), you will get access to the send/return feature.
however, you cannot actually use it with the S4 hardware on it’s own, as it lacks a proper in/out pair for this. You’d need to use different audio interface or a combination of audio interfaces … meaning, the S4 has master out & HP out only. you need an additional output to return the signal to an external mixer, as the send/return effect routing is geting the input from the external source, applies the FX, and outputs it on the return.