First and foremost...
Am I reading this correctly that you had two sticks and bought 2 more?
Or did you have a daul channel kit of 2 sticks then put those aside and bought a seperate 4 channel kit with 4 sticks?
If its the first and you bought two sticks, then bought two more this is a recipe for disaster and most likely your problem.
They sell them in kits for a reason. I wish it were different but its not. The faster the ram and tighter the settings the more important it is to buy the kit you need first time out.
When the sticks are made, they all do it the same way, every stick is plugged into a tester and its properties measured. They then bin sticks with like properties together. When they start making up kits they pull how ever many sticks and test them together before packing them up and selling them as a kit.
Mixing sticks from different kits and especially different manufacturing runs often leads to exactly what you are experiencing. Sometimes you get lucky and they happen to work but the forum is full of the same story in a massive amount of posts where sticks from different kits were mixed, even the same part number from the same manufacturer, and ended in less than disireable results. Some dont work at all, some wont run XMP or even OC in the least.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein