Coming from the proud owner of a i7-4930k @ 4 gigahertz;
I would have a hard time convincing myself to upgrade to a 4 year old cpu.
If you wait till Ice Lake you can get a 6 core i5.
Best case scenario here would be buy the Geforce 1070 and use it with your current rig and wait till Ice lake with its 6 core i5s.
Then buy ram, cpu and motherboard for Ice Lake.
There is also a case for leaving your current rig as is and building a whole new computer around an i7-7700k or Ice Lake coming soon since the only thing you can really salvage is the psu.
An i7-4790K is $390 on newegg ... money much better spent on a more recent cpu.
https://www.newegg.com/Product/Product.aspx?Item=N82E16819117369
Ironically an i7-7700K is only $340 so you pay a markup on the i7-4790k just to own older technology.
https://www.newegg.com/Product/Product.aspx?Item=N82E16819117726&cm_re=7700k-_-19-117-726-_-Product
You obviously can't use an i7-7700k with your current rig.
I was simply mentioning it as a price comparison to show how buying a 4790k is a bad idea.