If Serato DJ connects to the Xone DB4 or DB2 but you get no audio, or missing waveforms while using a Windows computer, you may be running into a known issue related to the latest mixer firmware.
If issues still persist, you may be running into this known firmware issue. The latest Mac OS X / macOS firmware for these mixers is not compatible with Windows machines. If this specific firmware is installed Serato DJ will connect, however, the mixer will not output any audio. In this situation, you will need to downgrade to the older firmware which you can find links for below;
DB2 v1.0.8 firmware - here
DB4 v 1.3.9 firmware - here
Any issues installing the firmware should be reported to Allen & Heath directly.