Using the left-hand endpoint, the approximate area is f(a)*(b - a) = 5*4 → 20
Using the right-hand endpoint, the approximate area is f(b)*(b - a) = 13*4 → 52
Using the midpoint, the area is (f(a) + f(b))/2 * (b - a) = (5 + 13)/2 * 4 → 9*4 → 36
(Not clear why you mentioned 4 rectangles!).