PLAAF serial system - the old 4-digit ones !??

A lot has been written about the 5-digit PLAAF serial system and even about the changes before and post 2005. But does anyone know, how the old 4-digit system works ???


An example are these early MiG-15bis and J-5's with only four numbers !

Thanks a lot in advance,
Deino


PS: Besides that is Rick Kamer still around ??? ... even better, can anyone arrange a contact with him ?? PLEASE !