MP3 is the format that taught music to travel: small, scrappy, and welcome absolutely everywhere. It was born in 1993, Fraunhofer Institute, Germany, and its real weak spot is holding fine detail at low bitrates: it stays small by quietly discarding sound.
MP4 is the world's default video: the file that plays before anyone has to think about it. A container, most often carrying H.264 or H.265 video with AAC audio. That makes it the right destination when what you need is playing everywhere: browsers, phones, TVs, editors, slide decks.