GTA IV was officially released for PS3, Xbox 360, and PC. It was never made for the PSP hardware.
Stick to official ports or verified homebrew from reputable communities like Reddit's PSP Homebrew. 2. The Mod Enthusiast Showcase (Style) Headline: How to get the "GTA IV Look" on your PSP Gta 4 Psp Zip
Grand Theft Auto IV was never officially released for the PlayStation Portable (PSP). The hardware simply wasn’t powerful enough to run that game. GTA IV was officially released for PS3, Xbox 360, and PC
While many sites claim to offer a file, there is no official version of Grand Theft Auto IV Gta 4 Psp Zip