Convert MOBI to MP4

Converting Mobipocket eBooks directly to MPEG-4 Videos (.mobi 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.