We only list the motherboard who had been tested on our QVL, and if you considering about the Asrock Z87 Extreme4 with single 8GB size memroy, we will suggest 1866Mhz memory kit.
If you considering about 2133Mhz for Z87 Extreme4, single size 4GB memory will be the only choice, and you could choose our 4GB x2 or 4GB x4 memory kits.
The performance memory kit(1.65v) needs more power to stabilize the system(higher frequency/timing/capacity)
One more thing for reminder, always use one memory kit instead of mixing two kits at the same time, it may cause incompatible issue. Please note!
If you still have questions about finding correct memory kit, let me know and i will list the compatible kits for you.