Replacement drive for DX4000

I’ve been looking for a replacement drive to have on hand for our DX4000-16tb, but it seems like all of the HDs that are listed as compatible (Answer ID 9443) for the device are discontinued or out of stock with no indication of when they would be in stock again. Are there any alternatives that do not require replacing all four drives in event of a failure?

Looks like the WD store has them ?