In a function, f(4) means that we are finding the value of the function when the input is 4. This means that we are evaluating the function at x = 4 and finding the corresponding output value.
On the other hand, finding x when f(x) is 4 means that we are solving for the input value that gives the output of 4. This means that we are finding the value of x that makes the function equal to 4.