How can I Convert M4V slideshow to MP4

.m4v is essentially the same as .mp4 (.m4v has just some added digital rights management options) so just changing the suffix to .mp4 should make it.

On the other hand .mov is a somewhat different wrapper. ffmpeg can losslessly convert .m4v to .mov:

ffmpeg -i input.m4v -c copy output.mp4

That does not preserve metadata but this should:

ffmpeg -i input.m4v -c copy -movflags use_metadata_tags -map_metadata 0 output.mp4

...unfortunately ffmpeg has a very old issue which incorrectly writes metadata (GUI applications usually use ffmpeg under the hood so they misbehave the same) so you must then fix it with exiftool so Photos.app can read it:

exiftool -m -overwrite_original -api LargeFileSupport=1 -Keys:All= -tagsFromFile @ -Keys:All output.mp4

Catharine Cary wrote:

thank you. I changed the suffix m4V to MP4 and the video can still not be read my quicktime player or VLC. It keeps opening Apple TV, which does not offer me an option to play it either.

Obviously TV.app is set as the default application to open files with that suffix. You can choose a better app for that:

Select the .m4v movie and then Finder > Get Info > Open with: and set the desired app like QuickTime Player.app open it by default -- click Change all... to make it the default for all files with the same suffix. Repeat for .m4v, .mov etc if necessary. For example I have set IINA.app to open .webm movies because QuickTime Player can not open them.

You can also do it once manually by opening a desired app via control-clicking the movie and "Open with" QuickTime Player or some other app that is listed there:

Handbrake will convert most things to most other things:

https://handbrake.fr/

And the price is tight too.