Because there is no official version, any "ISO" or "CSO" file found online labeled as Resident Evil 4 for PSP is typically one of the following: Fan-Made Mods:
In 2010, a developer managed to brute-force the Resident Evil 4 demo (the PC port) via and a video buffer hack. It ran at roughly 12 FPS with no sound. It is unplayable, but the ISOs exist. If you find a "working" file, it is likely this tech demo, not the full game. --- Telecharger Jeux Psp Gratuit Resident Evil 4 Psp Iso
When you search for that file, 99% of the links lead to one of three dead ends: Because there is no official version, any "ISO"