Asus — I want to upgrade my ram to from 2GB to 6GB

Address: Kolkata, West Bengal

I want to upgrade my ram to from 2GB to 6GB using another 4GB ram with my other 2gb but when i put in the 4gb ram the pc wont boot ... i mean it powers up but the screen stays black, PC does nothing ... when i try only with 4gb ram it still doesnt work, but when i try with the only the 2gb ram in my pc it runs ... why is that ? 4GB Ram 1333mhz ... and the 2 ram cards are the same model ... whats up with the pc not running with the 4 GB ram ?