Hi Oliver,
I think the processor got deconfigured during some reboot and may not have been done by your previous colleague intentionally. This may happen probably because of local hotspots, radiation and so on. You can check when your CPUs got deconfigured from MP>SL logs. They are not even classified as critical but as warning as sometimes manual intervention can cause it. Post the SL logs corresponding to 'CPU Deconfigured' message.
I also faced similar issue, but fortunately, after reboot, Processors got reconfigured. Having already rebooted, I suggest you to check if all 4 processors are configured now.
Regards,
Ram