Pioneer Dj DDJ-FLX4 Portable 2-Channel rekordbox DJ and Serato Controller (Graphite) - Manual (Page 42)

Connections
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3 Connect your PC/Mac or mobile device to the USB port (for device
connection) via a USB cable.
4 Turn on your PC/Mac or mobile device.
5 When using with a mobile device, connect a USB power adapter or
mobile battery to the USB port (for power supply).
6 Turn on the devices connected to the terminals (powered speakers,
microphone, etc.).