The Framework Desktop motherboard does actually have a PCIe x4 slot, their case just doesn't expose it for whatever reason. But you can buy the board separately and put it in your own choice of Mini-ITX case which does.

Or a Beelink GTR9 Pro with same chip and memory, and two 10GbE LAN ports built in for clustering several together.

In their article, AMD chose to use the built-in 5Gbe NIC, but I read you could use the two USB4 @40Gbps ports as interconnects too (or for USB NICs).