This hardly covers everything in act 4 but it does have some insight and analysis of selected parts. It is notes, not a polished essay, so don't expect it to read like one. It is intended to be read by someone already familiar with the play, and might not make too much sense otherwise.
Numbers seem important to the witches as they cast their spell. Maybe this shows how precise and deliberate they are with all their gruesome ingredients? They do not seem at all disturbed by what they are doing, yet they are fully aware of Macbeth's evil. Hecate appears to find joy in the disgusting task, as she compares their actions to elves and fairies. The witches sense the evil of Macbeth's prescence before anything else about him. This shows how much he has changed since his first meeting with them. The 'pricking' might be in reference to negatave effects Macbeth's prescence has on others because he has become so evil.
Macbeth calls the wiches evil, yet seems to have forgotten his own wrongdoing for the moment. He is so obsessed with finding out his future that he has no fear of the witches and their powers. Although he is led to think that Macduff is no danger, he resolves to kill him anyway just to make sure. Upon finding out that it is too late, he is already short-tempered from learning that Banquo's descendants will rule. He therefore looses any rational thought he had left and decides to kill anyone associated with Macduff.
Lady Macduff is distraught when she learns that her husband has fled. She will not listen to reason, and instead goes on about what must be wrong with him that has made him leave. Ross is in the uncomfortable...