Add the two extra 4GB sticks.

It’s cheaper, it works, and you won’t notice any performance difference vs. 2x8GB. Just make sure you buy the same model or at least matching speed/timings.

Four sticks still run in dual channel — the motherboard splits them into two pairs. You won’t lose any speed. The only real downside is you’re using all four slots, so upgrading beyond 16GB later means throwing away those 4GB sticks. But if 16GB is enough for this PC’s lifetime (and for most people it is), save the money and hassle.

If you know you’ll want 32GB eventually, buy 2x8GB now and add another pair later. Otherwise, just add the two 4GB sticks and be done.