Press the CUE buttons for both the MASTER channel and the channel you are mixing in (eg. CH4 is live, you are mixing-in CH1, you would cue MASTER + CH1). You can then use the cue mix knob to adjust the balance between the two sources in your headphone.
To hear only the mix and not the track pre-fader, press only the MASTER cue button.
My suggestion to achieve proper levelling, set the channel gain (for each track) so it peaks around the 0dB on the channel meter, take the channel fader to the top, make sure the crossfader is fully open or disabled, then adjust the master output level until the meter also shows 0dB. This way when you have another source cueing in the headphones at the same level on its channel meter, the volume will be the same between the master and cue when the mix is at 12 o'clock.