You can not use 4 sticks of any RAM on the Combo MB, there are 4 slots of which you can install 2 sticks DDR2 or 2 sticks DDR3. You can't use both types at the same time
From MSI specs
Main Memory
• Supports two unbuffered DIMM of 1.8 Volt D...
From MSI specs
Main Memory
• Supports two unbuffered DIMM of 1.8 Volt D...