Files with m4x file extension can generally be found as source codes from SynDEx.
M4X file extension - SynDEx data
What is m4x file? How to open m4x files?
File type specification:
The m4x file extension is related SynDEx and used for some sort of source file.
Most likely some old format without support from developer. No further information could be found.
This m4x file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.