In general there is only one important difference to know between the two formats mp3 and mp4. The mp3-format is only made for music or voice recordings, you can´t integrate or play a video or a picture with the mp3 format. The mp4-format can be used for songs with a video, for other videos or even for a whole film.
While MP3 means “Motion Picture Expert Group Audio Layer III” is only used for compressing audio files to create a new file which will need less storage volume, mp4 is used for the compressing of video files but it can also be used for the compressing of graphic files and audio files. Through cutting out noises which can´t be heard by the human brain the compressing saves a lot of storage volume.