DDR4 is not coming until Haswell-E... at the earliest... December 2014 and more likely sometime early in 2015.
As usual, be careful what you wish for. The main benefit of DDR4 will be lower power consumption for mobile applications and higher density modules (at a price premium). It's architecture also supports only one stick per channel (no impact on Apple there). As usual, the benefits for desktop users will be minimal and the costs will be higher (for early adopters). The fact is that with the monstrous L3 cache Intel equips it's desktop processors with, memory bandwidth does not contribute significantly to performance one way or the other so the benefits of going from 1833 to 2133 will be nearly immeasurable.
You can read all about it here...
http://www.extremetech.com/computin...-offer-ddr4-support-up-to-eight-cores-in-2014