What M4V to MP4 actually writes
An .m4v file from an Apple app will not upload to a site that only lists MP4.
The video is re-encoded as a standard MP4. A store rental or other DRM-locked file will fail because the browser cannot unlock it.
Quality and the M4V file you should keep
If the M4V is already a normal H.264 file you own, a desktop remux avoids the quality loss of this re-encode. Use this page when you need a file that simply plays.
A related step, if you still need one, is MP4 to M4V.
M4V to MP4 without the extra promises
M4V to MP4 sits on this site because M4V and MP4 are not interchangeable, even when they look similar in a file list. An .m4v file from an Apple app will not upload to a site that only lists MP4. A few Apple apps ask for the .m4v ending even though the video is the same family as MP4. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
The video is re-encoded as a standard MP4. A store rental or other DRM-locked file will fail because the browser cannot unlock it.
Read this about M4V before you upload
M4V is an Apple MP4-style video name, sometimes used by older iTunes workflows. Store rentals and other DRM-locked files fail. This does not add or remove FairPlay.
Keep the original if it already plays.
Read this about MP4 before you send the download
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
Keep, check, then delete
People lose time when they convert before they know which rule failed. Size, pixel dimensions, codec, font encoding, and layout are different rules. M4V to MP4 only addresses the M4V to MP4 step. An .m4v file from an Apple app will not upload to a site that only lists MP4. The video is re-encoded as a standard MP4. A store rental or other DRM-locked file will fail because the browser cannot unlock it. If the M4V is already a normal H.264 file you own, a desktop remux avoids the quality loss of this re-encode. Use this page when you need a file that simply plays. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. Do the test with a short or simple file first when the real file is large. A one-page PDF, a single photo, or a one-minute clip will show you whether the output is the kind of file you needed. After that test looks right, run the real file once. Keep both the M4V original and the MP4 download until the receiving person or the upload form accepts it. Video files hide the real problem inside the container. M4V to MP4 re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for M4V to MP4. You start with M4V and you stop when a real program accepts MP4. A standard file you own can be re-encoded to a normal MP4 or written back out with an M4V name. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
If the M4V is already a normal H.264 file you own, a desktop remux avoids the quality loss of this re-encode. Use this page when you need a file that simply plays.
A related step, if you still need one, is MP4 to M4V.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.