iMovie project save to File as MP4, MOV, or MPEG-4?

By default iMovie saves video files as .mp4

There is no significant difference between .mp4 and .mov etc. except that .mp4 is more universally playable, so you should stick with that unless you have some peculiar personal reason for not wanting to.

iMovie exports to Mp4 at all settings except Best Quality pro res, that yields a .mov file. Mp4 is a high quality compressed format that also is highly compatible and editable to a reasonable extent. The .mov file is much less compressed, better for professional editing, is higher quality, but yields a 4x larger file size than Mp4. The best format to choose depends on your purpose. Uploading a .mov file to the internet or website might be unwieldly due to the large file size, but would be more suitable for finite professional editing or where quality is of paramount importance. — like scientific displays or for large screen, or taking photo screen shots. Storage could be an issue as .mov files take up a lot of space. Mp4 is a more compressed delivery format that is very popular and plays pretty much everywhere with high quality, and would be more convenient for storage, sharing, or internet use. In other words, a bit more versatile and easier to work with.

For home movies and most other purposes Mp4 is fine. I personally have seldom had the need to use a ,mov file, but then I do mostly home movies and not professional use.

— Rich