There's at least one reported case of a PCIe multiplier card working with the Pi4: http://labs.domipheus.com/blog/raspberry-pi-4-pci-express-it-actually-works-u

There's at least one reported case of a PCIe multiplier card working with the Pi4: http://labs.domipheus.com/blog/raspberry-pi-4-pci-express-it-actually-works-usb-sata-gpu/ http://labs.domipheus.com/blog/raspberry-pi-4-pci-express-it...