Each channel in your motherboard has two memory slots.
Channel 0 is memory slot 1&3 (from right to left).
Channel 1 is memory slot 2&4 (from right to left).
With two memory sticks to get dual channel you need to put them in slot 1&3 or slot 2&4.
With four memory sticks you just fill all slots and you have dual channel memory.
A couple of caveats about running 4 memory sticks.
Make sure you are using the same brand and model to improve stability and performance.
Also take in consideration that running 4 sticks of memory stress the memory controller more than 2 sticks of memory, so you might have to change your overclocks (speed and/or voltage) if you are running any.
I'll also had that unless you have a specific need for those 16GB of ram, 8 GB is currently more than enough to run games and average daily usage.