Turn off the processor box? At the moment they have some working telephony but we don't know what's wrong. If the HD tray is faulty they will not be able to restore service!
Reboot on 4 is what - module test? So faulty processor is possible. But they are hot swappable.
I would remove the processors from ADP and standby SWU. Swap the ADP SIM card over to the other processor, and try to boot ADP. So same thing, but you don't need to power off, Active processor will remain running. If it still loops at 4, there could be another module causing the problem so then you are looking at a parts swapping exercise, maybe the MCM is dragging it down.