Comentário oficial
Hey Donnie,
Channels 3/4 are for either a line or phono input for an external source, or for the PC, which must be in PERFORMANCE mode. You can switch on the fly, just press the LINK or USB to select that source, then load a track. Just be aware that the beat sync data is not shared between the two sources, so you'll have to manually match both tracks.
If you wanted to use rekordbox in EXPORT mode, only channels 1 and 2 would be available for that.