Pop'n Music 4: Append Disc

Pop'n Music 4 features new songs and needs Pop'n Music 2 to work. An Append Disc is a disc that cannot be run on its own, and instead must be loaded through use of another disc (a Key Disc). In other words, you have to load the Key Disc first, then select a special load option (in that game), to be able to play the Append Disc.