MODULES MUST BE ORDERED AND INSTALLED IN IDENTICAL GROUPS OF FOUR for Quad Channel mode.
Mixing x4 and x8 modules is allowed, however not within the same Quad Channel bank group.
JEDEC spec "KSM" Server Premier Registered DIMMs will operate at 4800MT/s installed in 1DPC (1 DIMM per Channel) in Quad Channel configuration using Intel Xeon W9, W7 and W5 series processors. Due to processor/chipset limitations, 4800MT/s DDR5 RDIMMs installed in 2DPC (2 DIMMs per Channel) in Quad Channel configuration are limited to operate at and will safely clock down to 4400MT/s. Xeon W3 series and W5-3423 / W5-3433 models limit memory speed to 4400MT/s, regardless of DIMM configuration.
Refer to motherboard manual and QVL for supported memory configurations and speeds.
Per ASRock, S3 sleep mode is only supported with 1, 2, and 4 DIMM populations.