Line inputs 3 and 4 can be used on channels 3 and 4 and will work even while VDJ is using channels 1 and 2. Here’s what to check:
Make sure the input selection for channels 3 and 4 are set to “LN3” and “LN4”
Check that the PHONO/LINE switch for inputs 3 and 4 are set accordingly. If connecting a phono source (i.e. a turntable) with this switch set to Line, your signal will be very low.
Check that channel gain and faders are turned up and crossfader at the center.
Other than this, you may want to check that your source is properly sending a signal.
There is a switch on the back of the unit. it is used to set what signal is sent to the USB output (i.e. your computer). Here’s what the manual says:
LN3/4 THRU TO PC output mode
Audio signals input to LINE3/4 are output directly to the PC without passing through the mixer of this unit. In this case, the audio signal is not output to the master audio even if the channel input source select
knob is set to LN3/4.
So flip the switch to the Rec out/Mic setting for standalone use of line 3/4.