f(0) is equal to 4 in this graph.
You probably have seen the equation
at some point. f(x) is another way to express y. Essentially, f(x) represents the output you will get from an equation when a number equals x.
In this case, the input is 0, hence why it is notated as f(0). When x is at 0, y is at a value of 4. To further illustrate, f(3) is equal to 3, as with an input (x) of 3, the y value sits at 3, which means the output is 3.