Answer:
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.
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