Well, what I said is theorically possible, but chances are that it doesn't exist yet.

Have you seen the two-part adapters for mining rigs like this one? https://www.amazon.com/Mining-Express-Adapter-Extension-Ethereum/dp/B079SL7YDW

They include small internal PCIe 1x Cards (No I/O bracket) that goes into a standard PCIe Slot, which have only an USB Port (Not true USB, since it should be just the physical interface, data traveling though it should be in PCIe protocol). You connect that Port via an USB Cable to an independent PCB that has an input USB Port and a PCIe 16x Slot, and the PCB also has power inputs to feed the PCIe Slot power pins. Basically, instead of an ugly ribbon cable or powered one, you use that kit which is cleaner, but it is worth pretty much a single PCIe lane and I'm not even sure if signal integrity allows for PCIe 3.0 speeds or has to run at 2.0/1.0 ones.
Now, replace the PCIe 1x Card with either a 4x one or a 16x one with 4x/4x/4x/4x bifurcation for 4 ports, replace USB Port and cable connecting both parts with standard OCuLink, maybe add some redrivers for signal integrity, and you get my idea.