Find an O(h^2) formula for f'(x)+f''(x) by adding the midpoint formulas. What value of h should be used to keep error less than 10^-4?

I created the formula and have for my error term

h^2 /12 * [f^(4)(c1)+f^(3)(c2)]

How can I find a value for h when I don't know the value of the fourth and third derivative?