Do not use a generic Japan BIOS. Use the SCPH-39001 (USA) or SCPH-50001 BIOS for best compatibility. The beta checks for specific hardware flags.
If you own a physical copy of Gran Turismo 4 (any region), you morally justify the download of the Beta as a "derivative preservation backup." Gran Turismo 4 Online Public Beta Iso- - Google
Gran Turismo 4 Online Public Beta (ISO) refers to a rare, specialized version of the classic PlayStation 2 racing title. Originally intended as a temporary test for online multiplayer infrastructure, this build has become a focal point for the preservation and modding community. Historical Context and Purpose Released in , well after the retail launch of Gran Turismo 4 Do not use a generic Japan BIOS