Look at forums dedicated to PSP ISOs or fan-made conversions.
In the late 2000s, Capcom released a highly downgraded Resident Evil 4: Mobile Edition for iOS and older mobile platforms. Some homebrew developers have attempted to wrap or port these assets into an homebrew app format that runs on PSP homebrew architecture, though these are often buggy and incomplete. How to Play Authentic Resident Evil Experiences on PSP downloads resident evil 4 psp iso hot
Disclaimer: Ensure you own a legal copy of Resident Evil 4 before downloading or creating an ISO file. Downloading copyrighted content is illegal in many regions. Look at forums dedicated to PSP ISOs or fan-made conversions