Macbeth meets the three witches in both Acts I and IV. At the first meeting with the witches in Act I, Macbeth is greeted by the witches using a royal title.
All hail, Macbeth! hail to thee, thane of Cawdor! (Act I, iii, 51)
Macbeth knows, at this point, that he will be king. It is only through Lady Macbeth's murderous plots that he is able to obtain the crown and prove the witches' prophecy to be true.
I conjure you, by that which you profess, Howe'er you come to know it, answer me: (Act IV, i, 50-51)
Macbeth seeks out the witches in Act IV so as to find out where they received the information of the initial prophecy. He also wants to know what will happen from this point on. Macbeth knows that he will have no sons to pass down the crown to. He also knows that the prophecy names another as successor to the crown and he worries about the circumstances of passing/taking of the crown.
The difference between the two meetings with the witches is that during the first meeting, Macbeth has no true interest in the crown. It is only after he has murdered to gain the crown that he worries about keeping it.