Sonnet M.2 4×4 PCIe card

Sonnet M.2 4×4 is a full-length, full-height PCIe card that can host up to four M.2 SSDs, which can be combined for extremely high performance. The card offers an X16 interface to the PCIe 3.0 bus with x4 bandwidth for each M.2 2280 NVMe SSD, promising extremely high performance via software RAID configurations in appropriate host systems, such as the 2019 Mac Pro. A single (non-RAID) SSD can function as a boot drive. The card is compatible with Thunderbolt PCIe expansion chassis, though maximum performance is restricted by Thunderbolt’s bandwidth limits, and similar restrictions apply to PCIe 2 slots.

Sonnet M.2 4×4 [Amazon link] is due to ship Jan. 2, 2020, list-priced at $399.99 (sans SSDs) for host computers with appropriate PCIe slots running macOS 10.13 and later, Linux Kernel 5 and later, or Windows 10.

OWC Accelsior 4M2

OWC Accelsior 4M2 is a PCIe card from Other World Computing Inc. (macsales.com) that hosts four M.2 NVMe SSDs. Each SSD socket offers full x4 speed for maximum performance, and the full-height, half-length card handles PCI 3.0 at x8 speed in a 2019 Mac Pro or other computers (including slower PCIe 2.0 systems, such as classic Mac Pros).

You can boot off a single M.2 SSD, while the bundled SoftRAID app can create RAID 0, 1, 4, 5, 1+0 (10) volumes (sans support for FileVault encryption) for speeds “over 6,000 MB/s” (RAID-0, SoftRAID, two cards) with up to 8TB per card. Other features include “silent cooling via a finned heat sink cover”, an ASMedia ASM2824 controller, 380TBW endurance, and Trim support (OS-dependent).

OWC Accelsior 4M2 is priced from $479.99 with 1TB (Toshiba TLC flash) to $1,599.99 with 8 TB.  (Apple charges $2,600 for 8TB storage in a 2019 Mac Pro.)

ThunderBay 4 Mini

ThunderBay 4 Mini is a multi-drive storage system from Other World Computing Inc. (macsales.com) that connects to a computer via Thunderbolt — both Thunderbolt 3 models and Thunderbolt 2 models (compatible with Thunderbolt 1) are available.
Features include four 2.5″ drive bays with slide-in trays (for 6Gbps SATA SSDs or hard drives), dual Thunderbolt ports, a compact aluminum enclosure with “32.5dB cooling fan”, and SoftRAID software for combining drives in a variety of RAID configurations for improved performance and reliability (although SoftRAID is not currently compatible with FileVault 2 encryption or Fusion drive configurations).
  ThunderBay 4 Mini is priced from $299.99 as a bare enclosure with SoftRAID Lite to $2,069.99 with four 2TB SSDs and the full SoftRAID XT software version, for macOS 10.13 and later.