If we take the central difference twice we get $$\frac{f(x+2h)+f(x-2h)-2f(x)}{4h^2}$$
It is said that taking the forward difference and then the central difference is equal, but I do not get the same expression
$$\frac{f(x+2h)-f(x+h)-f(x)-f(x-h)}{4h^2}$$