If Disc 4 refers to a specific part of a game compilation or a chapter within the game:
The PSX Data Center confirms that the 4th disc of the limited version features a blend of exclusive content: 2.1. SD GGeneration Action Game (Wonderswan Port) SD Gundam - GGeneration-F -Japan- -Disc 4- -Pre...
: The fourth disc contained a hidden marvel—a complete, playable action game. This was a port of the Wonderswan Color (WSC) game, SD Gundam G Generation: Gather Beat . If Disc 4 refers to a specific part
The main 3-disc campaign adapts 38 different Gundam works . It spans from the original 1979 anime up through Gundam X , Turn A Gundam , and niche manga/video game spin-offs like Gundam Sentinel and Crossbone Gundam . Turn A Gundam