Can I add different size RAMs to iMac 5K?

I have an iMac Retina 5K (27-inch, Late 2014) with 8 GB of RAM (2 * 4 GB 1600 MHz DDR3 memory modules). There are 2 memory slots free/available, and I want to add RAM to these two slots.

My question is - Am I required to add only 4 GB memory sticks into these empty slots, or is it perfectly fine if I add a higher capacity RAM (8 GB sticks) in these empty slots?

I don't want to swap out the existing 4 GB RAM sticks. I want to keep these two as is, and add two more RAM sticks of 8 GB each.

I want to do this if I can be perfectly sure this works, and does not affect the system performance/reliability in any way.

I reached out to Apple customer support, and the customer service agent told me he wasn't sure, and that I should just stick to 4 GB uniformly across all slots.

Any of you good people have tried this already and have some advice?

Thanks

Yes, like on every other computer you can mix the ram sizes.

Though watch for the frequences (they have to be at same or higher) and the compatibilites.

The newer imacs support dual channel what means that tou can have 2 different sorts of ram memory. But make sure that there are at least 2 of the same installed so: 2x 4 gb and 2x 8gb should work fine. For more info, this link