After installing the latest Cumulative Update for Windows 10 KB3194798, when you open the disk cleanup tool and click on ‘Cleanup up system files’, the tool reports you can able to free up to 3.99 TB of disk space when the tool deletes Windows Update Cleanup, which is wrong and this is a definite bug, which you don’t need to worry.
It may take time for Microsoft to acknowledge this. No! You don’t have a hard drive or SSD, more than of 3.99 TB size, that’s you and me, others seen this can confirm for sure. Here built-in Windows Disk cleanup tool has done the calculation wrong.