What is f(4) is says to determine the input of the function when x=4.)

f(4) is called function notation.

typically you are given a function and need to evaluate it. As I believe you are trying to state, the 4 will be the input. See the example.

Step-by-step explanation:

f(x) = 3x + 4           This called a function, it has an input (the x) and a output

                             which is f(x). The question might look like this:

f(x) = 3x + 4,   find f(4)

What this is basically saying is: find the output, f(x), when the input is 4

f(4) = 3(4) + 4

f(4) = 16

For the function f(x) = 3x + 4, the output is 16 when the input is 4

Hope this helps