It's most definitely slots 1&3 or 2&4. We can look a little deeper and find the manual that can confirm this, but RAM slots are neatly painted the same colour for dual-channel RAM installation (as can be seen on the official website).
TL;DR Put those sticks in the same-coloured slots.
And I'd just like to add that you can check whether your RAM is actually running in dual-channel mode or not by downloading CPU-Z and clicking on the Memory tab. You should see this.