A Composite Function, as the name implies, is when one function is dependent on another function or when one function is inside another function
If g(x) is equal to x -4 then the f(x) is 1/x because when you plug in g(x) in to 1/x you get the composite function 1/(x-4)
Note that the order of the functions is important. f o g means that the function g(x) is inside f(x). If it were written g o f it would be the opposite ( f(x) inside g(x)).