Depends on the configuration.
4TB and contains one drive means there is a single 4TB drive in there.
4TB and contains two drives could go two ways - either it contains 2x2TB or 2x4TB. The first gets you extra speed (if RAID 0) or lower price (if JBOD), the second gets you extra reliability (if RAID 1).
https://getprostorage.com/blog/understanding-raid-storage/
Then the rest is just questions about your risk tolerance. There's no real functional difference between getting a single unit with two 4TB drives in a RAID 1 in it vs. getting two separate single-drive 4TB units and then manually copying your stuff twice, once to each. Both of those methods would provide essentially identical protection against single drive failure.
--Patrick