That platform can have a maximum of 8GB of RAM installed.
Refer to the Sun System Handbook.
(1) 280R page in the SSH:
http://sunsolve.sun.com/handbook_pub/Systems/SunFire280R/SunFire280R.html
(2) specifications:
http://sunsolve.sun.com/handbook_pub/Systems/SunFire280R/spec.html
(3) documentation:
http://www.sun.com/products-n-solutions/hardware/docs/Servers/Workgroup_Servers /Sun_Fire_280R/index.html
There are eight memory slots on the systemboard.
DIMMs are installed in quads (four same-sized modules at a time).
Refer to Chapter 2 in the system's Service Manual 806-4807 (10mb PDF file)