At a massive premium with very little performance gain
Really, though?
The voltage needed to run DDR4 is going to be a lot lower than DDR3 (will only need 1.2V or lower) which is a benefit and will provide more longevity and the clock speed will start at 2133MHz and eventually rise to 4266.
I bet everyone was saying the same thing with DDR2 and DDR3 when the latter was approaching the shelves.