Hello, on the EZ Menu when entering the UEFI (F7) check the top right of the screen. There should be a drop down to enabled REBAR. Once that is active simply check it is working in the OS.
This should enable 4G Decoding. Depending on GPU, you may need to also update the VBIOS.
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090