Windows Media Player or Media Player Classic typically support M4V.

If you are having trouble playing the video on certain devices (like a smart TV or an Android phone), you have a few easy options:

The VLC Media Player is a highly recommended free, open-source tool that plays almost any M4V file regardless of your operating system. Managing and Converting the File

If this is a personal recording from a smartphone or camera, it likely does not have DRM protection and can be opened with most modern media players: Use QuickTime Player or the Apple TV app.

An file is a video container format developed by Apple, very similar to the more common MP4 format. It is often used for content downloaded from the iTunes Store and can sometimes include Digital Rights Management (DRM) protection to prevent unauthorized sharing. How to Open and Play the Video

If the file was purchased from iTunes and won't play on a non-Apple device, it is likely protected by FairPlay DRM, meaning it can only be viewed on authorized devices signed into your Apple ID.