I wanted to clear up MP4, I think you mean MP3
All that is , is probably music. It might not be music you would like, but its simply music.
When many of us were younger we collected Albums, Cassettes or CD’s, today Kids collect their music on MP3.
It COULD be Vidoes, but unlikely as video takes up alot more space than music does and usually does not mean a Flash Drive.
Its probably also an MP3 Player / Flash Drive.
Many of the Cheaper MP3 players (Not Apple iPods) look like Flash Drives