M4V has an unfamiliar extension that some upload forms and editors reject, and iTunes-store files are copy-protected. GIF plays and loops automatically in every browser, chat app and email client. An M4V file usually holds H.264 video with AAC audio. M4V is at home in videos for Apple devices and iTunes; GIF is the usual choice for short looping clips and reactions. GIF has no sound, so the audio track is dropped.
The defaults suit most files. Under Options you can set the frame rate, set the width and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine. A GIF covers at most 30 seconds, counted from the start point you set under Options (the beginning of the video by default), so a longer clip is cut there. The defaults are 12 frames per second and 480 pixels wide, which keeps the file a sensible size.