If you've downloaded DLC Collection 4 and cannot access Modern Warfare 3, please follow these steps:
- From the Xbox LIVE Dashboard, go to Settings and select System
- Select Storage
- Select Hard Drive
- Select Games and Apps
- Select Modern Warfare 3
- Find and select "Collection 4: Final Assault Marketplace Content"
- Select Delete
- Launch Modern Warfare 3 and you should now be able to access the Collection 4 content
This will delete a key that is required to download DLC Collection 4, but will not delete the content. The key can be re-downloaded from the Xbox LIVE Marketplace if you need to re-download all of the DLC Collection 4 content at a later date.