There is no official PSP release of Resident Evil 4. A standalone PSP Resident Evil game was once announced but never released, eventually evolving into Resident Evil: Revelations for the 3DS. Fan-Made PSP Ports & Mods
If you want to play Resident Evil on your PSP, the only official way is through PS1 Classics download resident evil 4 psp highly compressed
These are typically fan projects where assets have been reduced in quality to keep file sizes low (often under 500MB) for easier downloading on mobile data. How to Download and Install There is no official PSP release of Resident Evil 4