I have added a new 4TB HDD, but only 2TB is online is disk management, how do in un-split (or un-partition), so that I can get it all online as one?
See attached screenshot of disk management.
It's because you formatted the drive with NTFS. NTFS will only see up to 2TB on a single partition in Windows. You need to use GPT format to have all 4TB on a single partition.