This tool reads your M4V video directly in the browser and encodes a MP4 copy, so there is no upload, no queue and no waiting for a server. It works on desktop and mobile.
M4V is an Apple video container and often has restrictions on non-Apple players. Converting to MP4 makes the same video playable on any device.
Protected iTunes purchases cannot be converted; this works for your own, unprotected M4V files.