Ok for the mk4duo I tested with 4.4.0 and it worked but had some problems with M117 so I needed to add
<bugfix name="FORBIDE_N" command="M117" />
to the firmwares/MK4duo.xml file right after "<busyHandling>relaxed</busyHandling>". Then M117 worked again without problems.
Connecting to a RepRapFirmware on a duet had nor problems. Did you have a printer with that or was that just a test for you mk4duo?