You'll get a few more fps swapping from the 4590t to the 4670, the biggest difference coming from the move from a 950 to a 1060. What's going to suck is that you are hoping for better, and not going to really get stronger performance moving from an i5 to an i5 and trying to play GTA:V. That game is miserable for quad thread cpus, doesn't matter if they are Haswell or Kabylake or AMD, 4 threads is the absolute minimum requirements and you'll still get close, if not at, 100% cpu usage.
Regardless of other benefits, the best move to alleviate seriously high cpu usage in GTA:V is the 4770 or 4790 or K version of either. The hyperthreading allowing upto 8 threads will considerably free up cpu resources and allow for higher fps, which you won't really get much of on a quad thread cpu.
And that's inherent to many games, not just GTA:V. The Xeon e3-1270/1280 v3 would also be a viable upgrade.