How to convert .docx files to .mp4 format.
Converting Microsoft Word Documents directly to MPEG-4 Videos (.docx to .mp4 conversion) is, per se, not possible. You might need to do a small workaround.
Using dedicated text-to-speech software, such as Balabolka or Narrator, you can certainly create an audio file (e.g. MP3) made of computer generated speech extracted from a text document.
Then simply convert the resulting audio file to an MP4 video, which is quite straightforward. Check out the following entry on converting MP3 to MP4.
A text-to-speech software
A text-to-speech software for Mac