It is quite obvious that it will not make a difference in games but this is the same thing as said before that happened when ddr3 came out and it did not make a difference but as time went on it became a standard. I would not get ddr4 yet may be after 2-3 years when it becomes cheap but for now you are good with a DDR3 Ram
Currently only Haswell-E supports DDR4, there are no motherboards for the 4790k that will support DDR4 as the chip itself needs to support it which it does not.