The "standard edition" of Forza 4 came with two discs. Disc 2 contains substantial "DLC-like" content (car packs and tracks) that can be installed directly to your hard drive or a formatted USB:
⚠️ The Xbox 360 Marketplace for Forza Motorsport 4 has been closed since 2021. The only way to get DLC now is from backup sources (e.g., your old hard drive) or game-specific archives for modded/jailbroken consoles. This guide assumes you have a modded Xbox 360 (RGH/JTAG) or are using a USB drive with Horizon (PC tool) for transferring DLC files you already own. forza motorsport 4 dlc download usb