Apparently there is a bug that shows 3.99TB of Windows Update Cleanup when you run the disk cleanup application in Windows 10. With the plethora of updates that Microsoft has thrown at everyone, this would seem about right, however, you can’t clean 3.99TB from a 1TB disk.
Although I’m sure my disk is OK since the entire internet is reporting this. It’s a good time to run a chkdsk just to be sure if you receive this. The odds are that the app is reporting the 3.99TB falsely.