4GHz is a bit ambitious especially on the stock cooler.
As for voltages, thats something youll have to tweak slowly on your own, there is no set voltage nor would auto be a good option. There are plenty of OC guides for that processor, it was decent for its time, but now not so much.
The same as Haswell I guess. The often quoted safe max overclock is under 1.4v core and under 72 degrees. source A max overclock would not be possible with a stock cooler. I would not go over 1.35volts core. You should be able to get 4GHz with much much lower voltage.
Reaching 4.0GHz should not be a problem at all! Some current chips do take a little more voltage, up to 1.2v, to reach it at these settings and my BIOS settings below is only a guide to what you may have to apply in order to reach the same as me. The Pentium G3258 for the most part does not need anything more than its stock cooler and stock voltage to reach a stable 4.0GHz overclock.
"the miracle chip". Well you are in luck, as in this guide I will cover an assortment of overclocking topics, all of which are focusing on the best results and how to achieve them. In the past Haswell chips have been known for its low overclock results past 4.2GHz. It has been said only about 70% of current chips can reach 4.5GHz.