It is 1x, obviously. But I highly doubt that the PCIe bus is going to be the bottleneck here ;-)
I guess it would be possible to design an ATX size motherboard with a RPi4 compute module slot (once CM4 becomes available).
The PCIe bus would allow all sorts of peripherals. Don't expect any performance though.
Well, yes, something like that, but not in a cluster form, but just a single CM4 on a mini-itx form factor board, with PCIe slots, USB ports, ethernet, etc.
You could build a fully blown Linux desktop peripheral wise, just don't expect desktop performance.
I guess it would be possible to design an ATX size motherboard with a RPi4 compute module slot (once CM4 becomes available). The PCIe bus would allow all sorts of peripherals. Don't expect any performance though.