Kojima Productions public relations lead Jiro Oishi revealed a secret about Metal Gear Solid: Peace Walker. Early in development the game was called Metal Gear Solid 5: Peace Walker.
The “5” isn’t there now, but it was flashed during the E3 teaser for Peace Walker and Metal Gear Rising. Perhaps, Konami dropped the 5 at the eleventh hour.
While this isn’t Metal Gear Solid 5, Oishi feels the content in Metal Gear Solid: Peace Walker is the apex of the numbered Metal Gear Solid games.