Ok let me explain it in car terms. Consider your GPU as the engine of the car. You put in a 450 HP V8 engine in your car and pair it with a Ford Figo tyres. Now your engine is capable of more speeds but your tyres aren't allowing it to get maximum performance. So your tyres are bottlenecking your engines performance.
Similarly if you pair a high end GPU like the GTX 770 with a low end CPU like a 4100, some games where the 770 is capable of giving more fps, because your CPU isn't up to snuff you will get lower fps.
This will generally happen with modern demanding games. Less with older games.
Regarding upgrading your CPU please make another thread for that.