Mixing 4GB and 8GB RAM memories in a late 2013 iMac

I have an iMac (27-inch, Late 2013) with 4 slots for RAM memory. I upgraded it changing the 2 original 4GB memories for 2 x 8GB memories. Is it possible to install back the original memories to work together with the new ones? 2x4GB + 2x8GB. If it's the case, In what order in the slots?