What does Macbeth plan to do in Scene 4 of Act III after Banquo's murder?

At the end of Act III Scene 4, after Macbeth has learned that Banquo is dead but Fleance escaped, then behaved like he was a sandwich short of a picnic when the ghost of Banquo showed up at his banquet, thereby deeply disturbing all the lords in attendance, Lady Macbeth dismisses the guests ("Stand not upon the order of your going / But go at once."). Macbeth, still disturbed by the gory ghost of Banquo (who has left the scene), says, "It will have blood; they say, blood will have blood," then muses about how odd things happen in nature (trying to make sense of what he has seen). 

Macbeth then says: "How say'st thou, that Macduff denies his person / At our great bidding?" That is, "What do you think of the fact that Macduff, who was invited to the banquet, didn't show up?"

Lady Macbeth responds oddly, saying, "Did you send to him, sir?" This is an odd question, in my view, because until now, Macbeth has conspired with his wife, but this tells us that she didn't know about his orders to have Banquo killed, which means she didn't understand what he was raving about at the banquet. She must have been as freaked out as his guests. 

Macbeth responds, "I hear it by the way; but I will send," meaning he's heard rumors of why Macduff didn't attend, but he'll check into it. This is again odd. Why doesn't he tell his wife, who started this trail of bloodshed, about his orders to have Banquo and Fleance murdered? But he keeps the facts close to his chest. He merely responds, "There's not a one of them but in his house / I keep a servant feed," meaning that he has spies in every great house in his kingdom. 

What does he plan to do next? Go back to the weird sisters, of course. He is now "bent to know / by the worst means, the worst. For mine own good, / All causes shall give way: I am in blood / stepp'd so far that, should I wade no more, / Returning were as tedious as go o'er. / Strange things I have in head, that will to hand; / Which must be acted ere they may be scann'd."

This marks a deep psychological change in a man who was, in the beginning, "too full of the milk of human kindness / To catch the nearest way" to the crown. Macbeth, without consulting his wife, let alone being coerced by her to commit evil deeds, has decided that he will return to the witches (understood to be against nature and God's law, so he's daring hell here) to find out what lies ahead for him, and he will know, no matter what he has to do. He has realized at this point that he has gone so far that it doesn't matter if he slaughters a few more people; he's already a murderer, so what's a few more murders in the grand scheme of things? He resolves to act on his plans (to retain his crown) before he has a chance to think about them from here on out. He has shut off his conscience now.