.mov is quicktime's native format, therefore it will, by default, always save as a .mov file. If you want to save it as a .mp4 file, just choose "Export" instead of "save".
MOV is not one file format but rather a format envelope that can be made up of any of dozens of codecs, resolutions, and frame rates from basic DV through H.264, HEVC to ProRes, and other pro formats.
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