Pioneer Dj DDJ-FLX4 Portable 2-Channel rekordbox DJ and Serato Controller (Graphite) - Manual (Page 44)

Connections
44
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 Connecting to an audio output device (example 2)
In this example, the master sound and the headphones monitor sound
both play in mono.
(1) Speaker with built-in amplifier, etc.
(2) Headphones
(3) Plug for the speaker with built-in amplifier, etc.
(4) Plug for the headphones
(5) Split cable (not supplied)
(6) USB/Lightning to 3.5 mm headphones jack (not supplied)
When connecting via Bluetooth, you'll need to make the correct setting
in rekordbox for iOS/Android.
 Read more: Starting rekordbox for iOS/Android (page 109)
(6)
(2)
(1)
(3) (4)
(5)