The PPSSPP is an emulator specifically designed for the PlayStation Portable (PSP), a handheld console from 2004. Forza Horizon 4 is a graphically intensive title designed for Xbox One and modern PCs. The hardware gap is too wide; the PSP never had a version of FH4, and therefore, no legitimate "PPSSPP ISO" exists for it.
If you are committed to using the PPSSPP emulator and want a high-quality racing experience similar to the Horizon vibe, try these official PSP titles: Massive car list and great physics. forza horizon 4 ppsspp file download
I can point you toward a safe, working game that fits your style. The PPSSPP is an emulator specifically designed for