@geerlingguy postedhttps://www.jeffgeerling.com/blog/2023/building-tiny-6-drive-m2-nas-rock-5-model-b really interesting concept of super compact 6 drive nvme rock 5B addon called mini nas.
Unfortunately it’s first and last time I’ve heard about this project. Jeff had some issues with this device and it was designed for 5B (no plus). You may ask Jeff or Silvertip Labs for progress.
Easiest way to get four nvme running with RK3588 is to get SBC from Friendly Elec, this will have four nvme slots, each one with 1 pcie 3.0 lane. You may connect two more nvme’s via usb (not that recommended) or replace one m2. bottom slot with 6x sata card (this requires much bigger setup and additional power) or penta sata kit on top (should work but may require longer cable, this will solve some power issues).
Technically it should be possible to design bifurcation card that convert two m.2 to four m.2, same like CM3588. If I really need to keep it compact, I would just exchange radxa Rock 5B+ to that.