Ryzen is particularly fickle about memory. So considering that for ANY system or platform, mixing sets or sticks of memory that didn't come together in a matched set and was tested for compatibility at the factory, is always a crapshoot. Even more so with Ryzen. This could result in a variety of things from as little as, nothing, to they won't play nice together at all, to they just won't run in dual channel together.
Additionally, a lot of systems WILL NOT run four sticks of memory without some additional tuning. On just about any system, running four sticks may require increasing the memory (DRAM) voltage, applying a CPU overclock, increasing the VCCIO and/or system agent voltage, fiddling with speed and timings. In some cases, depending on the platform, motherboard and memory being used, they simply will not work at anything above the default JEDEC serial presence detect (SPD) settings which for DDR4 means 2133mhz. I've seen systems that just wouldn't work, at all, with four sticks, even when they came in a matched set that included all four sticks.
Considering the fickle nature of Ryzen's memory compatibility and depending on what memory you have to start with, as well as how good the motherboard is, this could even be more problematic than on an Intel system. Both Piledriver and Bulldozer had memory issues of their own, and had a hard time with memory over 1866mhz or using four modules, so this is not a new problem for AMD even though they are not alone in sometimes having issues in this area.
The memory speeds you see supported by any given motherboard and CPU are generally intended to apply only when using two sticks of RAM. They may, or may not support those specifications with additional modules installed.
I would HIGHLY recommend that your best and most likely way to have success with this is to either buy two 16GB sticks, or buy four 8GB sticks in a quad set that has been tested together. You can always sell the memory you have now, or use it in another system. That all being said, you CAN try what you are about to try, but even if you buy identical part numbers there is every chance you will not get memory that is exactly the same and that can typically cause issues. Case in point.
http://www.tomshardware.com/answers/id-3610013/amd-ram-compatibility.html#20562100
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