What an .m4v file actually is
M4V is Apple's video extension, and a non-DRM .m4v is essentially an MP4 — the same ISO base media (MPEG-4 Part 14) container, usually the same H.264 video and AAC audio inside. Apple shipped the extension so iTunes and the Apple TV ecosystem could mark files as their own and so FairPlay DRM could ride along on Store purchases. Strip the DRM question away and the difference between a clean .m4v and an .mp4 is mostly the four letters after the dot — which is exactly why so many apps and upload forms reject a .m4v even though the bytes play fine. They whitelist the extension, not the codec.
What this tool does, exactly
Native ffmpeg decodes the M4V stream, re-encodes the video to H.264 at CRF 23, re-encodes audio to AAC at 160 kbps, forces pixel format to yuv420p for maximum decoder compatibility, and writes the result as an MP4 with the +faststart flag so it begins playing immediately when streamed. It is a single-file, no-options conversion — no trimming, cropping, batching, or quality knobs. Because a non-DRM M4V is already MP4 underneath, the practical effect is a clean, standards-correct .mp4 that the apps which choke on the .m4v extension will accept.
The honest caveat: FairPlay DRM cannot be removed here
Many .m4v files from the iTunes Store — movies, TV episodes, and music videos you bought or rented — carry FairPlay DRM. This tool cannot convert a DRM-protected M4V; a re-encoder cannot strip FairPlay, and circumventing it is out of scope. What this tool converts is non-DRM M4V: files you recorded, exported, or otherwise own that simply landed with Apple's extension. If your .m4v plays in QuickTime without asking you to authorize a computer, it is almost certainly non-DRM and will convert fine. If it demands authorization or refuses to play outside the Apple ecosystem, it is FairPlay-locked and out of reach of any honest converter.
Sister tools
Going the other way or working with neighbouring formats: MP4 to MOV converter wraps your video for QuickTime and Final Cut. MOV to MP4 converter handles QuickTime, ProRes, HEVC, and iPhone sources. AVI to MP4 converter and MKV to MP4 converter cover legacy containers. Shrinking a finished MP4 further? Video compressor.
Skipping the conversion entirely
If you keep ending up with .m4v files from exports or screen captures and constantly converting them, consider recording straight to a standard, shareable format instead. Clipy records your screen directly to a shareable link — no local file, no extension mismatch, no format conversion, no watermark, no install.