Here is a blog post covering what you need to know about the Resident Evil 4 PPSSPP experience. Survival Horror on the Go: The Resident Evil 4 PPSSPP Guide

Go to the official website (ppsspp.org) and download the emulator for your device:

Many search results claim there’s a Resident Evil 4 ISO for PSP. That’s – Capcom never released it for PSP. What exists are:

Since there is no official ISO, you have two primary methods to experience Resident Evil 4 using the PPSSPP emulator.