What does Hamlet call King Claudius at the end of Act 4?
King Claudius seems to be helping Hamlet steal away to England in an effort to protect him from the consequences of killing Polonius. Hamlet agrees to go to England by saying "Good." The scene unfolds as follows:
King Claudius - So is it if thou knew'st our purposes.
Hamlet - I see the cherub that sees them. But com, for England. Farewell, dear mother.
King Claudius - Thy loving father, Hamlet.
Hamlet - My mother. Father and mother is man and wife, man and wife is one flesh, and so my mother. Come, for England.
Not only is Hamlet insulting his uncle, but he continues to assert his power in the relationship the only way he knows how--by contradicting everything his uncle says, does, and is. Hamlet's fatal flaw is his indecision to act and just kill Claudius. This small interchange shows Hamlet's immaturity and indecision. By contradicting his uncle, he shows his disapproval on every turn, but he is not quite powerful enough within himself to do the deed. The insult may also be a rhetorical way to emasculate Claudius thereby showing that Hamlet does not recognize Claudius as strong enough to be a king, but he had to marry a woman in order to gain the crown. The power-play between the two certainly drives both men to devise ways to get rid of the other.
When Hamlet exits in Act IV scene 3, after killing Ophelia's father Polonius, he calls Claudius "dear Mother." Thinking he has simply misspoken, Claudius tries to correct him and says, "Thy loving father, Hamlet." However, Hamlet has not misspoken, in fact he meant the insult towards Claudius. He explains to his uncle, stepfather, "father and mother is man and wife, man and wife is one flesh, and so, my mother.”
Part of this is a slam at Claudius and Gertrude's hasty marriage to King Hamlet's brother, but it is also a part of Hamlet's act to pretend to be crazy. By messing up names and speaking in a doublespeak, Hamlet's confusing those around him and truly beginning to make them feel that he is crazy.