I thought MP4s are audio files while JPG are images/pictures. Is that correct? Are you trying to extract a picture from an audio file?
If so, how about playing the MP4 on your device (presumably a laptop) and stopping playback when you reach the image you want to copy. You should be able to use "ctrl-prtscr" or the Windows "snipping tool" to copy the image to the clipboard. You could then paste the image into a program as simple as Microsoft Paint and save the image from there to JPG format.
Sorry if this isn't what you are trying to do.