the old version of windows movie maker can't open MP4 files recorded by most of cell phone, and if you google for solution, you will found many junk website , they propose complicated ways and trial or paid software, but the simple easy and powerful is the windows movie maker
some website propose even WMM 2012 like new version 2016 and ask for money don't trust that, latest version is 2012
Movie Maker was officially removed for download on January 10, 2017. Unlike other Windows Essentials apps, Movie Maker has not yet been replaced or succeeded by anything else. Microsoft stated a new version in the Windows Store would be available "soon". However, once they pulled the download on January 10, they removed references to the promised new edition. With no official first-party download and no confirmation of a new version, Movie Maker has effectively been discontinued.
Movie Maker is included in the Windows Essentials 2012 program suite so make sure that you update your framework too.
SOLUTION you need just to download the latest version and this is direct link