He is a post form another site that explains why:
Re: AMD760 + 4DDR ... Where?! [re: skwerly]
Just use the biggest sticks you can find. Steve Benoit of Stable Technologies explained the reasons for two slots in much more detail. Concerning unregistered DDR DIMMS "the specifications for unbuffered RAM are 3 clock cycles per DIMM. This being the case, having 4-DIMM slots would require the AMD761 chipset to support 12 clock Cycles.... As this is like, well impossible." He went on to say "Registered DIMMs only use 1 clock cycle, therefore, using 4-Registered DIMMs would only require 4 clock cycles, which is what both motherboards were actually designed for in the first place."
So the use of unregistered DDR DIMMS precludes the use of 4 banks. They originally intended to release boards that would support unregistered in only two banks or registered in all 4 banks, but I think they opted for a wise choice in simply cuting back to two sockets, as most poeple would not understand what was going on and instead assume a defect which is not really present.
Video killed my Radio Card!
If it works for you then don't fix it.