Which of the following sets of ordered pairs is not a function?

{(1, 4), (2, 5), (3, 6), (4, 7)}
{(0, 2), (1, 2), (2, 2), (3, 2)}
{(-1, 5), (2, 4), (2, 6), (-3, 7)}
{(-4, 3), (-1, 1), (-2, 5), (3, 7)}