In Act 4, what does Macbeth do that seems to ensure his tragic downfall?

Actually, his tragic downfall began when he murdered
Duncan.

In Act IV, scene 1, he consults with the witches
who have been instructed by Hecate to mislead him because he interfered with the
predictions by killing Banquo and attempting to kill Fleance.  This they do by filling
him with over-confidence.

In Act IV, scene 2, since he
cannot get Macduff, he trys to neutralize him by killing his wife and children.  This
action is perhaps the most heinous of all.

Act IV, scene 3,
seems to seal Macbeth's fate when Macduff goes to Malcolm and offers his services.  With
the help of the English king, Malcolm is able to raise an army and invade Scotland to
take his rightful place as king.

All these actions lead to
Macbeth's eventual downfall.