For the third order differential equation, y = f (x, y, y , y ), where f (x, y 1 , y 2 , y 3 ) is Lipschitz continuous in terms of y i , i = 1, 2, 3, we obtain optimal bounds on the length of intervals on which there exist unique solutions of certain nonlocal three and four point boundary value problems. These bounds are obtained through an application of the Pontryagin Maximum Principle from the theory of optimal control.