starter, home basic, and home premium support 1 physical processor. professional, enterprise, and ultimate support 2 physical processors.
32 bit versions support up to 32 logical processors. 64 bit versions support up to 256 logical processors.
EDIT: the only boards you're going to find that support four physical AMD processors are going to be server boards (opteron) in the $800+ range
, and you'd have to invest in a server operating system to utilize them.