Hello,
Use the supplied cables and ensure all 4 8-pin connectors are inserted. Ensure the 12VHPWR power connector on the 4090 is fully inserted.
Use an independent 6 pin to power the PD_12V_PWR on the motherboard.
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090