DDR4 will be useful for servers and perhaps cellphones, but it has no current value for typical desktop PCs as even DDR3 @ 1600 MHz. is not a system bottleneck. Each new industry standard is typically developed years before it's needed and this is no different. DDR4 has some significant departures from DDR3 and won't be rapidly adapted to the desktop for a number of good reasons, though server mobos will likely start using it in the next 6-12 months - depending on cost.
You will see more low voltage (1.35v) DDR3 as DRAM makers try to stimulate DDR3 sales because DRAM prices are in the cellar and staying their with PC sales down and a rocky economy expected for years more.