Without this step, even with the files on your USB, Forza Motorsport 4 will not load the DLC cars.
However, as of October 2023, the Xbox 360 Marketplace shut down permanently. This means you can no longer download Forza Motorsport 4 DLC directly from Microsoft’s servers. So, what do you do if you get a used copy of the game or replace your hard drive?
Now for the moment of truth:
If you try to search for Forza Motorsport 4 DLC on the Xbox Marketplace today, you will likely come up empty-handed. This is due to a phenomenon known as "delisting."
Anyone successfully gotten FM4 DLC onto a USB drive lately?
Understanding the timeline is crucial. Forza Motorsport 4 used the now-defunct Xbox 360 Marketplace. While you can still re-download items you already purchased via your Download History, you cannot buy new DLC. Furthermore, if your Xbox 360 hard drive fails or you purchase a pre-owned console, Microsoft’s authentication servers often fail to recognize older licenses.
While hunting for "Forza Motorsport 4 DLC Download USB" links, be extremely cautious.