To identify if the board could be in culprit try another CPU as well.The memory controller is in the CPU ,so if you have issues with memory you have 2 parts involved.Check the CPU installation ,check for bent pins as well ,move the 4G sticks in all slots on the motherboard.
Also you could try the following ,maybe your RAM is over-clocking stuff and requires more volts to start.
With the 2giga sticks (the ones that work) up the voltage in BIOS for the memory ,save restart ,turn off and then install the 4 giga sticks.For DDR 3 the auto voltage in BIOS should be 1,5 Volts ,you can try 1,55V and then 1,6 Volts to see what happens.
Also it could be the case that the ram you are trying is simply defective.
Please post the name of the memory modules that work and the name of the ones that don t.
Also post the name of the power supply and your full system configuration as in the sticky in the AMD section.Maybe the few watts the RAM you are installing adds are over the limit for your power supply.