I'm having a hard time understanding these kinds of problems.

A function associates x and y values. The graph of the function is formed by all the points such that x and y are actually associated, i.e. y=f(x).

So, if you choose x=4, you can see that the correspondant point on the graph has a y coordinate that is somewhere between 3 and 4 - much closer to 4 actually.

So, an estimated value could be around 3.75