As bignastyid said, the performance difference going from DDR3 to DDR4 is negligable. I wouldn't lose any sleep over it. A 4790 with some decent DDR3 will perform very well.
Just store that RAM in a safe place and buy some DDR3 RAM, LGA1151 CPU/MOBOs support DDR4. Unless you're using the integrated graphics on your 4790, you won't notice a difference from DDR3-DDR4.
As bignastyid said, the performance difference going from DDR3 to DDR4 is negligable. I wouldn't lose any sleep over it. A 4790 with some decent DDR3 will perform very well.