Another issue I have come across when routing a mono signal in my DJ mixer (Model1), I believe it might be similar for the DB4 (according to the manual): as there is no pan and no normalling of a mono input to L+R, you may need to use a splitter cable (TS => 2×TS; not a Stereo breakout cable) to route the mono signal to both L and R sides of your stereo track on the mixer.