The MP4 player is, essentially, the sequel to the MP3 player. MP3 players played MP3 files (songs), but the MP4 player does much more.
MP4 players do play MP3 files. Many MP4 players play many other song files as well, along with MP3 files.
The MP4 player is able to play even more files, including pictures and video, while maintaining the same basic song functionality as an MP3 player.
One upgraded feature that the MP4 player offers is video capabilities. The MP4 player allows you to play back different types of videos.
The MP4 player also offers picture-viewing capabilities. Many MP3 players did not offer this.
There are many companies that sell MP4 players, with the bigger-name companies in 2010 being Apple, Microsoft, Sony and SanDisk. When making a decision, you should consider how much memory you will need and how easy the interface is to use.