<< " The Handbook does not definatively specify whether this upgrade is possible " >>
http://sunsolve.sun.com/handbook_pub/Systems/SunFireX4100/spec.html
http://sunsolve.sun.com/handbook_pub/Systems/SunFireX4100/components.html#CPUMo dule
The information on those pages seem understandable enough.
Keep in mind, that if you install modules from third-party vendors,
you suspend your warranty for all the time they're inside the chassis.
That means no techsupport from Sun, even for other hardware components.
You would need to use cpu modules from Sun to keep the warranty in force.
It all depends on the CPU "socket" on the motherboard. AMD's CPUs use many versions of CPU sockets for their chips. Some sockets are upgradeable, some are not. For example, the x4100M2 system has a CPU socket for its dual core chips that WILL accept the upcoming quad core chips.
You should consult the AMD web site for CPU socket compatibility. They have a matrix of supported CPUs by socket.
My gut guess would be that since the 4100 comes with both single and dual core CPUs, the motherboard would be identical for both and that an upgrade to dual core would be just a drop in upgrade. But consult the AMD site for the best information.