If Forza Motorsport 4 is not available through official channels on PC, you might consider:
: A community effort called Project Plus provides improvements specifically for the Xenia emulator, such as modernized tire models and unlocked DLC content. Official Alternatives on PC
Forza Motorsport 4 was released exclusively for the Xbox 360 and was never officially ported to PC. Any website claiming to offer a "Forza Motorsport 4 PC Free Download" is likely distributing malware or fraudulent files, as the game does not exist in a native PC format.