For those of you who are unfamiliar DDR4 RAM for a long time has not been consumer grade RAM and has been too expensive to purchase. Well as of PAX it has officially hit mainstream and is now much more affordable.
Why would I care about DDR4 RAM over DDR3 RAM? Well power wise DDR4 and DDR3 are basically the same, but DDR4 RAM is much more efficient putting a lower strain on your power supply and can even help to lower the overall temperatures of your rig.
Edit: Oh and the mainstream CPUs that support DDR4 RAM are the Intel skylake processors which include the i5 6600k and the i7 6700k which also recently hit the mainstream as well.