You might run into address space issues. I haven’t checked Broadcom PCIe documentation for RPi4 (is there any?), but I tried a very similar hack with i.MX6 and

You might run into address space issues. I haven’t checked Broadcom PCIe documentation for RPi4 (is there any?), but I tried a very similar hack with i.MX6 and older AMD and nVidia cards. They get recognized fine, but BARs cannot be mapped because they don’t fit in i.MX6’s tiny 16MB PCIe space.