The DR-40’s four-track recording mode gives you several options to record, indicated by LEDs on the front of the unit. The 4-channel recording modes include:

DUAL – Record a stereo signal with a copy of that stereo signal at a lower level. This gives you a safety track you can use in case the main recording is distorted. (See next section for more info)
4CH – Record the built-in microphones along with a pair of external mics, like close-up and ambient microphones. Or record the built-in mics along with a balanced line input, such as the feed from a mixer along with the sound in the room.
OVERDUB – Play along with a recording you’ve already made to build up a full production or song demo
You can record up to four channels of 96kHz/24-bit audio. Mono and stereo recording is also available. Recording formats include MP3 (from 32 to 320kbps), WAV and Broadcast WAV formats.