add. info after rereading the OS/4 kernel info text for OS4APIC.PSD:
In case the MP tables are broken, ACPI4.BPD will interact with OS4APIC.PSD to provide it with the equivalent info from the ACPI tables.
The MP tables as well as the ACPI tables provide OS Software with the information how many cores exist, how many and what type of interrupt controllers exist, etc.
The MP tables however are an outdated way of supplying this information and therefore, they are often broken/incorrect on modern systems. Today, every MOBO producer is strongly encouraged to support ACPI (as well as UEFI, but this is a different topic ...).
Therefore, I am not sure if ACPI4.BPD supports all the other ACPI stuff like power saving and shutdown. You will need to try out. I can say that on my system and with the aforementioned drivers, I can fully power off the system by selecting system shutdown.