Spending so much time looking for good routers for OpenWRT, or SBCs with multiple Gigabit Ethernet has got me wondering "How hard would it be to modifiy the Raspberry Pi 4 circuitboard to add another ethernet controller....".

But modify the board i mean, can I open it in KiCAD and make enough naive guesses to swap the USB controller for an ethernet one. I read that they're both on the secret PCI-3 bus.

So, perhaps its as simple as swapping out the USB controller package and replacing the traces to the USB port(s) with traces for an ethernet port instead.

I mean, the board even has an existing controller as an example I can copy!