It can be done (at least I had it working when I was using a controller about a year ago) but it needs a hardware volume control for the headphones.
Send either the left or right record output to one side of the headphones and Traktors monitor out set to mono to the other side.
Set the internal headphone volume to 0db (reset it by double clicking on the knob in software), this is so the volume of the record out and monitor out are equal.
Map the cue/PFL select buttons on the controller to also adjust the cue/mix control. If no channel is cued set it to mix (100%), if any channel is cued set it to cue (0%)- this will require modifiers to ensure it only switches back to mix when all channels are ‘de-cued’.
With the MC4000 I think you would need some kind of volume control between the controller and headphones to control volume levels (as there is no control of the record output level in the software). This depends on if the MC4000’s headphone volume knob is hardware of software based.