Lets start with the obvious , the sabertooth is a dual channel ramset so to use it properly there can only be 1, 2 or 4 sticks used . you need ram in the first slot or the first and third if using 2 sticks and all 4 slots if using more than 2 whatever you put in slot 2 wont work alone if your running 3 sticks . so Im going to guess if your only getting 3.9gb you have the first 2 slots with the 4gb in each and 2gb in the third . by doing this the second 4 is not working because its not dualed with anything and the 2gb is not working because its probably not compatible with the 4gb in the first slot. so the only stick thats working is whatever is in the first slot. which must be a 4gb stick .
Now are the sticks all matching as far as speed , timing ,latency and voltage . If not thats another problem
So I'm going to to assume the 4gb are the same if so put them in slot 1 and 3 and dont bother with the 2gb until you have another matching 2 to dual in slots 2 and 4