I see only one spare part number for empty shelf, so it is unlikely that it has any SAS/SATA specific settings. Did you try to power off shelf, remove two PSU and replace them with blanks, power on again? 4PSU SATA shelf is valid and I am pretty sure that if you remove PSU while shelf is powered on, it should report an error (how else would you know you lost it? 🙂 )