Wont notice any major differences between 4770 vs 4790. You will however notice a big difference with Hyperthreading in multi threaded games which most are nowadays.
Example of my own system 2600k @ 4.5 w/1080ti. 4 cores only (HT off) running Ghost recon or Fc5 cpu usage was 95%. With HT on cpu usage decreased to 60ish and gained 20fps more.
HT be cheaper alternative than a complete system overhaul and since modern games of late run well with Hyperthreading (unlike Dying Light in my experience) either your cpu choices will hold you steady for another couple years or so. Overhaul when Pcie4 or 5 is released.