The alternative is looking at the RAID drives for Mac like My Book Studio II. To get the 4T you would need to run as RAID 0 . The 2 drives are then seen as one. There is no redundant copy that way. If one drive goes bad you loose everything. In RAID 1 if one drive goes bad there should be a copy on the other drive.
I’m looking for the same thing. I have four of the 2T My Book Essential disks and I’ll buy four new ones if they are 4T or larger. I don’t want the dual-disk solution since it is way too expensive and bulky. The 3T disks are too small. It’s been 18 months since there has been a rev. WD had a good thing going. Hopefully they’ll get moving. And if not WD, then hopefully someone else will and I can buy from them instead.