There is no motherboard with DDR4 you can get for i7 4790k. DDR3 is the only RAM it supports.
That being said, DDR3 is perfectly fine, and as long as you have enough of it it won't hinder your performance. There are far more important factors that limit performance than just the type of memory - for example, an actual amount of memory is certainly more important than the type. Besides, your CPU is still plenty fast for whatever task you need (especially when overclocked).
TL;DR - you don't really need DDR4. If you really must upgrade RAM, just need 16GB of DDR3 in a matched dual channel kit of 2 modules. Better upgrade your GPU if it is slow, of get an SSD if you don't have one.