I would have said go with only two DIMMs on Ryzen AM4 platform, before today, based on my prior experiences.
But this morning, I finished re-building my main rig, with a Gigabyte B450 AORUS PRO WIFI, and I decided not to move my RAM over from my current rig (2x8GB GSkill DDR4-3000 CAS15 or CAS16, 1.35V), but instead, to use my two kits of 2x8GB Team Vulcan DDR4-3000 1.35V RAM. Previously, in an ASRock AB350M Pro4 board, I tried using two kits of 2x8GB Team Vulcan DDR4, one pair of kits was 2400, the other pair was my 3000, and the 3000 would only POST and run at 2400. (Edit: At first glance, it appears that I'm saying that I used one kit of 2400 2x8GB, and one kit of 3000 2x8GB, and mixed them. That is not the case. I had two identical rigs, one with two 2x8GB DDR4-2400 (32GB total, 2400), and two 2x8GB DDR4-3000 (32GB total, 3000, but would only POST at 2400 as well).)
But fast-forward to this morning, new Gigabyte Aorus B450 board, same Ryzen R5 1600 CPU, and I put in both kits of 2x8GB DDR4-3000 Team memory, set XMP, and... it not only POSTed, it booted Windows, and I haven't had any BSODs or appcrashes yet! It seems that recent mobos have licked the issue with Hynix RAM and Ryzen CPUs. This is still a 1st-gen Ryzen CPU too, not 2nd-gen, with the better memory controller.
I'm happy. 32GB of DDR4, running at 3000. (1.35V, according to CPU-Z, after setting XMP in UEFI).
So, this finally proves that you CAN run four sticks of RAM on Ryzen, at a reasonably high DRAM clock speed. (I don't own any DDR4-3200 yet. to test.)