Official comment
You can, and there's a couple of options for you. The quickest / easiest is to go to PAD FX and you'll see SLIP LOOP effects; those will do a roll which allows the track to continue to "play" in the background.
The alternative method would be to engage slip mode (by clicking the SLIP button onscreen; there is no hardware button for this on the DDJ-400), and then pressing the BEAT LOOP pad mode button. Each of the pads would then engage a slip-loop.