Now that Banquo is dead (though Fleance has escaped), Macbeth has to make some more decisions about how to proceed, how to keep power, and how to protect himself. He decides that he "will send" to get information from a spy he keeps in Macduff's house to find out why he "denie[d] his person / At [Macbeth's] great bidding" (3.4.162, 3.4.159-160). Apparently, Macbeth keeps a spy in all his nobles's houses. Macduff has failed to show up at Macbeth's big dinner party, and it is a pretty public snub, especially considering that he did not go to Macbeth's coronation in Scone either. Beyond this, Macbeth decides to go to see the Weird Sisters the next day. He says,

More shall they speak, for now I am bent to know
By the worst means the worse. For mine own good,
All causes shall give way. (3.4.166–168)

He plans to make them give him some additional information, as he wants to know the worst that's going to come his way. He will try to ensure that all obstacles be removed from his path.

Act 3, scene 4 finds Lord and Lady Macbeth presiding over a banquet in their honor. The two murderers return to tell Macbeth that they succeeded in killing Banquo, but Fleance got away. When Macbeth returns to the banquet hall he sees Banquo's ghost sitting at the table; he begins to address the ghost, unable to hide his anguish at seeing the ghost of his friend who he murdered at the table. The company is aghast at his behavior and Lady Macbeth attempts to excuse Macbeth's odd and frenetic behavior by blaming it on an illness that he's had since childhood. Macbeth finally manages to pull it together, but loses it again during the dinner. Everyone leaves and Lady Macbeth berates her husband for his behavior and for ruining the evening. 

Macbeth makes the decision at the end of the scene to send for Macduff in the morning and to visit the witches to find out how to protect himself. He worries that he might lose all that he has gained, but has come too far to turn back now.