Linux:How to protect mp4 file from copying but being able opened to play

On a Linux machine owned by you? Easy, force the user to log in as a user with no access to the shell, file managers, etc. They'll have no way to copy the file.

On a Linux machine owned by someone else? You'll have to use the concept of DRM, and unfortunately DRM on Linux is crippled by it's open-source nature. The kernel isn't locked down the way the Windows and MacOS kernels are, so you can't rely on the OS as the root of trust.

Will your next question be like "how to prevent screen capture"?