The DueX4 doesn't need any special configuration, it presents additional stepper drivers, hot ends and thermistor inputs that are configured using gcode just like the ones on the Duet.
To use the drivers on the x4 to drive axes, make sure you are using firmware 1.16 or 1.17rc1 and look up M584 in the gcode wiki on this site.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
