First thing I would do is make a copy of the MP4 file so that you can try various things with the copy and leaving original 'safe'. Have you got Windows Live MM rather than WMM? If WLMM have a look at this, http://texaslastfrontier.com/how-to-use-mp4-files-in-windows-live-movie-maker/
If Windows MM then try converting the file to AVI or .wmv using FormatFactory before importing.
http://format-factory.en.softonic.com/ , as with all freebies, use the custom install option, to remove the toolbars etc offered with installing. It's a very good converter.