In The Tragedy of Macbeth, Act IV, why does Macbeth take comfort in the second and third predictions of the apparitions?

When Macbeth first meets the witches, he doesn't think much of what they've said until he is given the title of Thane of Cawdor which was the witches' second prophecy. This encourages his ambition, and is already contemplating murder to get to the throne, but he can't force himself to do the deed himself, and is therefore encouraged by his wife. In the second set of prophecies Macbeth believes that he has nothing to worry about. The first apparition told him to beware Macduff, but the next two apparitions make him feel as if he does not need to worry.