You have three choices:
A) Plop down about R15k for a Core i7-5820K, a 140mm AIO water cooler, a Gigabyte X99-UD4 and 16GB DDR4-2400MHz and don't worry about upgrading for another +-six years
B) Wait for Skylake and only spend about R9000 that will last you +-4 years before you consider upgrading again.
C) Buy a better cooler, 8GB DDR3-1600 memory, a newer PSU and overclock to 4.5GHz to enjoy another two years before you seriously have to consider an upgrade. (you can stick with your existing memory, but 1333MHz RAM makes for some funny ratios/speeds when you start increasing the BLCK value)
As for the question on DDR4, it does make sense and it is faster, but you need to start at 2400MHz and up if you want to see any actual performance increases in games or applications which scale with memory bandwidth. Battlefield 4 is one of them.
Skylake will also be pretty spectacular. I'm expecting overall a 30% jump in IPC performance alone (compared to Sandy Bridge) at similar clock speeds and having access to all the modern memory, connectivity and storage standards would be nice.