We're starting to see a fair number of DDR4 memory options on the market, but DDR3 is still, by far, the go-to RAM. DDR4 is currently limited to Intel's X99 platform and Extreme processors, which are more expensive, and not much better for gaming, than the Core i5s and i7s we're accustomed to. DDR4 is still much more expensive than DDR3, but it also hits higher clock speeds. The question, then: how different is DDR4 from its predecessor, exactly? The folks over at AnandTech have gone to great lengths to answer that.
First off, DDR4 operates at a lower voltage than DDR3. DDR4 runs at 1.2 volts, down from 1.5. It doesn't sound like much, and it's really not for your typical home PC. Most Haswell-E desktop systems (where you'll most often see DDR4 in use) will operate somewhere in the 300W to 1200W range. The voltage difference for those numbers might account for a 15W savings over DDR3—not a lot for a home user. But for server farms and other large-scale computer architectures, where you could have hundreds of systems running thousands of DDR4 modules, that 15W difference adds up.
Another big difference between DDR3 and DDR4 is speed. DDR3 specifications started at 800 MT/s (or Millions of Transfers per second) and some went as high as 2133. DDR4, meanwhile, starts at 2133 MHz. The increased speed means an overall increase in bandwidth.
This unfortunately comes with an increase in latency as well, but the increased clock speed makes for quicker transfers while maintaining an overall latency comparable to DDR2 and DDR3. DDR3-1600 operated at a latency of CL11, which took 13.75 nanoseconds to initiate a read. DDR4-2133 sits at CL15 and performed a read at 14.06 nanoseconds—only a 2% increase.
So how does this translate to real-world performance? Should we all be eager for Intel's next chipset to move us all to DDR4? Well, not really. As Anandtech found in some comprehensive benchmarks comparing DDR3 and DDR4, neither was a clear performance leader. The difference between DDR3-2133 and DDR4-2133 was negligible in a number of applications, ranging from Handbrake video conversion to half a dozen different games. DDR3 was slightly faster about half the time, and typically only a few percentages points separated the two memory types.
The AnandTech article goes into significantly greater detail about the differences between the two memory standards. Check it out for more info.