If you've downloaded DLC Collection 4 and cannot access Modern Warfare 3, please follow these steps:
  1. From the Xbox LIVE Dashboard, go to Settings and select System
  2. Select Storage
  3. Select Hard Drive
  4. Select Games and Apps
  5. Select Modern Warfare 3
  6. Find and select "Collection 4: Final Assault Marketplace Content"
  7. Select Delete
  8. 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.