MP4 files: opening, M4V differences and conversion

MP4 is a multimedia container defined by ISO/IEC 14496-14. It can hold audio, video, metadata, interactive scene information and references to external media streams. MP4 names the container; it does not identify one fixed compression method.

Open the MP4 in your media application for playback. If it fails, check the application's support for the audio and video inside the container. For conversion, select the video as a source in HandBrake if installed; its documented source support includes common MP4 and M4V videos, but excludes DRM-protected video.

Try opening the file in QuickTime. For HandBrake-generated files with chapters or AC3 audio, QuickTime 7 and applications based on it require .m4v to use those features. QuickTime X does not have this extension restriction. That distinction does not guarantee support for every contained stream.

Open the file in the media application you use on iPhone. HandBrake documents that iOS does not have QuickTime 7's .m4v extension requirement for chapters or AC3 audio in its output. If playback fails, check the contained encoding and application support; the extension alone cannot establish compatibility.

For HandBrake output, MP4 and M4V differ only in their extension. HandBrake may choose M4V when AC3 passthrough, SRT subtitles or chapter markers are enabled. Use .m4v when the documented QuickTime 7 feature restriction applies. Renaming the file does not re-encode its streams.

The MP4 extension does not establish support for its contained streams. Check the intended application's encoding support. HandBrake supports common MP4 and M4V video sources, but does not support DRM-protected video or bypass copy protection. An opening failure alone does not prove file corruption.

It depends on the operation. FFmpeg streamcopy copies compressed streams without decoding or re-encoding, preserving the quality of those copied streams. Transcoding decodes and encodes again and can reduce quality. An MP4 output extension does not promise a lossless conversion.

Use streamcopy when the target container can accept the source streams. In FFmpeg, -c copy selects stream copying. This preserves the copied audio and video streams without re-encoding; it does not promise preservation of every container feature. Missing information required by the target container can make the conversion fail.

Use video/mp4 for general audiovisual content, audio/mp4 for audio without visual presentation, and application/mp4 for content with neither audio nor visual presentation, such as MPEG-J or MPEG-7 content. The correct MIME type depends on the contents.

RFC 4337 warns that malformed media streams can overload or crash a receiver or decoder. MP4 can also contain program content, though not every file does. MPEG-J restrictions do not remove decoder risks. The extension or successful playback is insufficient evidence that an unfamiliar file is safe.