What are the witches doing at the beginning of Act IV?
In Act III, the Weird Sisters were chastised by Hecate, the goddess of witchcraft, for, among other things, not including her in their dealings with Macbeth. She has told them what to do to ensnare Macbeth even further: to raise some deceptive spirits that will make him feel secure because "security," she says, "is mortals' chiefest enemy." If the sisters can make Macbeth feel safe in his position, he will let his guard down and leave himself more vulnerable.
So now, in Act IV, we see the witches conjuring the spirits they will show to Macbeth in order to give him this sense of safety and assurance. They cast their spells, throwing a bunch of disgusting things into their cauldron and ordering the doors to open for Macbeth. He demands more information about his future, and when the Weird Sisters offer to call their "masters," Macbeth -- with great bravado -- commands them to do so. The apparitions have been conjured, and they present enigmatically worded statements of truth that sound more like assurances of safety, fooling Macbeth just as Hecate hoped they would.
At the beginning of this act, we see the iconic quote "Double double toil and trouble" as the witches prepare some kind of brew in their cauldron. The witches describe all sorts of foul objects (such as a lizard's leg and and an owl's wing) being added to the brew, and many of these suggest dark symbolism. The witches also hint at the presence of other spirits who speak to them. They are waiting for Macbeth to arrive. When he enters the scene, he demands to hear the witches' predictions. They summon their "masters", three ghostly apparitions, who provide Macbeth with vague warnings against MacDuff, warnings that Macbeth does not ultimatley heed. Macbeth's questions lead the apparitions to dissipate without revealing more, but the scene ultimately foreshadows the events ahead.