Speeds above 4000MHz require Gear 2 which runs the Fabric Clock asynchronously. The best CPU performance is found running in 1:1 ratio with the FCLK (Fabric Clock). 4800 is also the maximum validated ratio for the Crosshair VIII. Even if wanting to run asynchronously with FCLK, I'd normally recommend buying a kit below the maximum validation for an easier time tuning.
I would aim for a tight 3600-3866 if the CPU will allow you.
13900KS / 8200MT CAS36 / ROG APEX Z790 / ROG TUF RTX 4090