I also thought it was supported when I chose to develop for IFC4 in the XML format. I assumed that if IFC4 was supported, that the XML sub-format (".ifcxml") would be included but I was wrong. On the 'About Revit and IFC' page for Revit 2020 it doesn't claim to be able to import or export IFCXML files. It does work with IFC4 step files (".ifc") though.
When you try to open or link an IFC4 XML file, nothing seems to happen, but a log file is produced in the same directory as the IFCXML file (filename + '.log.html') which includes an erroneous error saying, "Can't determine XML file schema.. Aborting import." I imagine the error would be relevant to an earlier version of IFCXML.
When you export an IFC4 XML file, Revit produces an IFC2x3 XML file without any indication that it didn't do what you asked it to do. If you open the IFCXML file that it produces in a text editor, you will see near the top that it references the IFC2x3 schema.
