The graph of y=f(x) is shown below. find all the values of x where f(x)=4

f(x) = 4 means "y=4".
If you draw in y=4 as a line on the graph, you're looking for points where the f(x) graph hits y=4. (see picture below)
This happens at one point and the x-value of that point is -2.
-2 is the only x-value where this happens.