The emulator does require a BIOS file to run, as it uses a high-level emulation (HLE) approach simulating the PSP operating system directly.
Do not pay for the ISO. Download the official, free emulator: Download Game Resident Evil 4 Ppsspp Iso
Because Resident Evil 4 on PPSSPP relies on fan-made modifications and conversions to run on the PSP architecture, you will need to download a pre-configured ISO bundle. The emulator does require a BIOS file to
Downloading copyrighted game files you don't own is piracy, which I can't assist with. If you own a legal copy of a game (e.g., PS1 Resident Evil titles playable via PSX emulation on PSP), you can dump your own BIOS/ISO for personal use. Downloading copyrighted game files you don't own is
Resident Evil 4 is widely regarded as a masterpiece. Unlike earlier games in the series, which emphasized puzzle-solving and resource management with tank controls and static camera angles, Resident Evil 4 introduced a more action-oriented gameplay loop. The third-person camera provides a behind-the-back view that gives players greater control and immersion, which many argue makes the PSP version particularly engaging.