DDR4 is next generation DRAM technology for use in computers. DDR4 will arrive in computers in late 2015 when Intel Skylake processor supports it. AMD will support DDR4 in early 2016. AMD needs to work on some new technology to fully utilize DDR4 speed. Now they lags behind Intel in RAM bandwidth utilization.
DDR3 data transfer rate range is 6.4 GB/sec to 24 GB/sec.
DDR4 data transfer rate range is 12.8 GB/sec to 25.6 GB/sec.
As you can see DDR3 can achieve data transfer rate that DDR4 promises. But in DDR4 there will be other advantages.