ExtremeTech discusses the state of DDR4 memory, you can read it over here. The site heard from Corsair that the first DDR4 chips will arrive by year-end, but these products will be targeted at servers.
The first DDR4 chips, he told me, will be coming out at the end of 2012 — but “the initial target platforms are servers, not desktops.” In fact, Pearce even acknowledged the rumors that DDR4 might never be seen on desktops at all, but went on to point out that “this is really something that will be dictated by Intel and AMD since they are the driving force behind the memory used in client computing platforms (based on the fact that the memory controller is integrated into the CPU).”
Watercooled RAMX-bit labs estimates that, if DDR4 does show up on desktops, it will probably be in 2014 and production ramp-up will be rapid, with DDR4 instantly garnering about 12% market share and increase to 56% in 2015. Given how long DDR3 took to catch on, I’m a bit skeptical about that, but it’s far too early to say for sure.