Are you filling the banks correctly? My SparcStation LX (also a sun4m) has to be done in pairs but the sockets are not adjacent.
http://docs.sun.com/app/docs/doc/805-4450/6j47ekk4e?l=ru&a=view
and
http://docs.sun.com/app/docs/doc/805-4450/6j47ekk4e?l=ru&a=view#z40001b74c5