I agree, what Zombie proposed will work. Make sure you put the 4GB sticks in the "first" slots (actually DO read the page in the manual of the motherboard because they are all marked a little differently, like A1, A2, etc) and put those 4GB sticks in the pair of slots that the manual says to use if you only have two sticks total. That will give you best performance.
Then, populate the remaining two slots with those 2GB sticks, and the computer will use up the 8GB on the two bigger sticks first (one on each channel), and then if ever it needs more, it will dip into the smaller sticks.