Reference DDR4 chips are at the exact same frequency that they will use with Xbone (2133 Mhz). The only difference is that 2133 effective clock frequency is the max frequency validated by JEDEC for DDR3, while it's the base frequency of DDR4 specifications. It doesn't changet anything either way.
Also DDR4 is nowhere near GDDR5 effective frequency, it's 2133 vs 5500 : x2.5 frquency and then bandwidth at the same bus width.







