At the end of the readme, the creator had written:
Simply put, . Rockstar Games never developed or released an official version of Grand Theft Auto IV for the PlayStation Portable. Gta 4 Psp Zip
Since you are looking for "GTA 4 PSP," this is your best bet. It takes place in Liberty City. While the graphics are not as photorealistic as GTA IV, the city layout will feel very familiar. It offers the same open-world freedom, driving, and shooting mechanics that the series is famous for. At the end of the readme, the creator