Oh well. Looks like a matter of time until these can be shrunk down enough.
Then again this SSD is so pricey, wouldn't it make sense to go with that 400-500$ 3rd party accessory board/controller instead that allows you to run 3 or 4 SSDs inside the machine?…
My OP was about a 4TB SSD. Those are averaging $800. So, a pair of 2TB @ $250 ea + a $400 board isn’t a bad deal. Especially when you consider that the 4TB blades are double-sided and will cook as surely as the 8TB did. This board cannot use double-sided blades either.
The center slot is for the original AHCI. If it doesn’t work without that, then functionality is limited. I haven’t researched enough if these require the center to be filled or not.
In any case, your boot SSD is a maximum of 2TB. Period. APFS is required as is High Sierra or later for an NVMe blade to perform in a bus designed for AHCI. You can’t boot from RAID 0 over APFS in HS. With 2TB being your maximum boot drive, might as well run external storage over Thunderbolt — I think the $400–$500 is better spent on that.
Moving on...
OWC did offer a 4TB ACHI blade for the 6.1. Like all other AHCI drives, no one makes it anymore. I recall it costing $2k or more but it got down to around $1,200 or so when OWC ran a clearance. I’ve not been able to find one anywhere at any price. Of course, like all AHCI, its maximum speed is around 60% of NVMe in a 6.1 — or any Mac not upgraded in 2015 (some Airs, Mini and a few others). The 2015 iMacs got the slow blade in an NVMe 3 x4 bus Leaving them perfect candidates for an inexpensive major upgrade — but I digress.