^what he said is not completely true.
Most motherboards are:
CPU
Channel A slot 1 (let's say red)
Channel A slot 2 (let's say black)
Channel B slot 1 (red)
Channel B slot 2 (black)
Each set of color has one Channel A and one Channel B. When they sync together, that's what makes them dual channel. So then, that means 4GB modules should be installed in one color. 2GB modules need to be installed in the other color.
Channel A slot 1 2GB (red)
Channel A slot 2 4GB (black)
Channel B slot 1 2GB (red)
Channel B slot 2 4GB (black)
So if you have a motherboard that does not alternate color, then it's:
Channel A slot 1 2GB (red)
Channel B slot 1 2GB (red)
Channel A slot 2 4GB (black)
Channel B slot 2 4GB (black)
or if they are all the same color, just make sure the initial dual channel slots are populated with the same modules (like A2/B2 - 4GB), and the secondary pair of dual channel slots are populated with the other kit. (A1/B1 - 2GB) Just keep in mind one slot from each channel A/B makes dual channel.