Two files end in .mp4. One plays; the other produces an error, a black picture or sound without video. The matching extension does not establish that the files contain the same kind of video and audio.
MP4 is a container. The streams inside use codecs, and the receiving app needs to support the relevant combination. MDN distinguishes container support from codec support. A familiar extension is a useful clue, not a promise that every device can play the file.
Separate a transfer problem from a format problem
First check whether the download or copy finished. Ask the sender whether their original plays, and compare file sizes if practical. Matching sizes alone do not prove that a file is intact, but a much smaller received file is worth investigating before conversion.
Next, inspect the media information in a trusted player or editor that can read the file. Record the container and both the video and audio codecs; a file-manager label may identify only the container. Check support for the recipient’s actual app and operating-system version. Apple’s guidance also distinguishes unsupported formats from damaged files.
Export a small sample before the full video
If conversion is needed, use a compatible export preset in an app you already trust. Confirm the preset against the destination’s requirements rather than relying on a name such as “MP4”. Renaming the extension does not re-encode the streams.
Export a short copy and have the recipient try it. Check picture, sound, orientation and playback through the end. Keep the original: a successful conversion can still change quality, subtitles or other details. Once the sample works in the intended app, export and inspect the full copy.