The single bay/single drive My Cloud has a feature called Safepoint (firmware v4.x) or Backup (firmware 2.x) in the My Cloud Dashboard. This feature can be used to backup the My Cloud to a USB hard drive attached to the My Cloud USB port. See the My Cloud User Manual at the following link for more information on how to perform a Safepoint / Backup.
It will take a very long time (as in hours) to backup 4 TB of user data. Depending on how the backup is configured the first backup is almost always the longest. Additional backups may go faster if only changed/new/deleted files are updated on the USB backup.