You'd need a carrier card with a PCIe bridge chip for using multiple M.2 cards within a single PCIe slot. I have found two such cards:
https://www.amazon.com/Aplicata-Quad-NV ... 1515673618This first card only has an 8x PCIe 3.0 uplink so right off the bat with four M.2 drives, you'll only have half the peak possible bandwidth. If the host system only has PCIe 2.0 slots available, uplink bandwidth is again cut in half which corresponds to the link used by a single M.2 slot. Yep, a single fast M.2 drive could monopolize the entire uplink bandwidth.
https://www.thedebugstore.com/squid-pci ... -gen2.htmlThe second card has an 16x PCIe 2.0 uplink so bandwidth is cut in half as well and the links to the M.2 slots operate at PCIe 2.0 signaling speeds.
The other thing worth noting that these cards need host system support for them to be bootable. Hardware RAID support again would have to be provided by the host system but considering the bandwidth restrictions, (8x host uplink, PCIe 2.0 host uplink respectively), at most you'd get the performance of two M.2 drives. Software RAID would be lower performance. Overall RAID here is something I would not recommend.