Q. I am running Disk Cleanup and see 3.99 TB of Windows Update Clean-up. What is this?
A. You can ignore this and you may not even have 4TB of storage. What is happening is Disk Cleanup is not understanding Hard Links which is a mechanism to allow multiple file objects to actually point to the same data on disk. What is being seen in this example is many links to the same data which is displayed as a huge amount of data. The reality is the actual amount of data is much less. You can safely delete the data. The actual data on disk is not deleted unless all hard links are removed so the data is safe.