Sun,
you need to look at your motherboard or MANUAL to see how they have the memory slots configured.
PAIR the 8 GIG sticks with each other and the 4 Gig sticks pair together.
Otherwise you will not get the dual channel.
This way you will get it on both sets of memory slots.
After checking your motherboard manual, your slots are configured and memory should be PAIRED as follows:
1 and 3, 2 and 4 . So I would put the 8 GB modules in 1 and 3, the other two in 2 and 4, assuming you really want to use ALL of those.
You will have a total of 24 GB of RAM.
Hope this helps!