DDR4 wouldn't help really AMD since they are using DDR3-2133 already. Initial RAM speed targets for DDR4 are not much faster than DDR3-2133. Next year we see max DDR4-2133 with Haswell-E and from what we heard Skylake supports max DDR4-2400. Given that the GPU Raw performance increases every year a switch to DDR4 doesn't change the bandwidth limitation.