How does Macbeth's attitude toward the witches in Act 4 contrast with his first encounter with them? What does this show about his character?
During and after Macbeth's first encounter with the Weird Sisters, he is incredulous. He, apparently, had never thought about becoming king (Duncan does have two sons, after all), and he has trouble believing anything the sisters say is true until the Thane of Ross tells him that he's been granted the title, Thane of Cawdor, as the sisters had predicted. Once this happens, Macbeth begins to look on the kingship has, essentially, a guarantee, and he is shocked and quite disappointed when Duncan names his older son, Malcolm, his heir.
When Macbeth next encounters the Weird Sisters, he has sought them out, unlike the first time he met with them (when they sought him). He longs to know information about his future, especially now that Macduff has slighted him and roused his suspicions. He trusts the sisters implicitly now, without question. In terms of his character, this shows how easy he is to manipulate; Hecate knows that "security is mortals' chiefest enemy," and everything she plans for him comes to fruition. He is a bit gullible, never pursuing questions about how the sisters get their information or even wondering what possible motives they might have to share this information with him.
At first, Macbeth was skeptical of what the witches had to say to him. While it was interesting for him to hear (in the same way a good horoscope or fortune in a fortune cookie would be) he didn't pay it much mind.
Now that he has taken it upon himself to become king and fulfill the other "prophecies" the witches have made, Macbeth is eager to hear more. It's almost as if the witches' prophecies give him an excuse for commiting horrible deeds.
Macbeth begins the play as a noble and loyal solider. He has now become power hungry and a believer in "black" magic.