Rhetoric is a language art derived from Ancient Greece which covers a range of techniques designed to make speech more persuasive and emotionally affecting.
Shakespeare was a master of rhetoric and Act IV of Hamlet features many interesting examples.
Firstly, we can see an instance of parallelism in IV.iii as Claudius hurries Hamlet off to England. As he explains that Hamlet's ship is ready to depart, Claudius creates a sense of urgency by placing similarly structured phrases in parallel with each other in lines 47 - 50:
Therefore prepare thyself;
The bark is ready, and the wind at help.
Th' associates tend, and everything is bent
Secondly, when Hamlet has departed, Claudius uses synecdoche (pronounced sinekdokee) to emphasise the Danish power over England. He describes the Danish army as "the Danish sword" (line 68), giving an image of a huge threatening sword held over England's "raw and red" scarred body. It seems that Denmark has inflicted military damage on England in the past, so Claudius is confident that the English king will do his bidding and execute Hamlet when Hamlet arrives on English shores.
Thirdly, as Hamlet begins his journey to England, he uses arhetorical question whilst he is alone to vent his frustration that he has still not enacted his revenge against Claudius. He asks himself
What is a man,
If his chief good and market of his time
Be but to sleep and feed?
It is not his intention that someone nearby should answer this. He merely uses it to express his exasperation that he is doing little more with his time than sleeping and eating.
Fourthly, in the same speech, Hamlet usesalliteration to further express his painful vexation that, despite the positive example of noble Fortinbras, he is unable to act for his own cause. Notice the gasping repeated "e" sounds at the beginning of lines 48 - 54, then the "m"s in "makes mouths", followed by the decisive "d"s towards the end to show his respect for Fortinbras's strength of character:
Examples gross as earth exhort me:
Witness this army, of such mass and charge,
Led by a delicate and tender prince,
Whose spirit, with divine ambition puff'd,
Makes mouths at the invisible event,
Exposing what is mortal and unsure
To all that fortune, death, and danger, dare...
Finally, later in this act, in IV.vii, Claudius uses isocolon to harness Laertes' aggressions towards Hamlet and persuade him to agree to killing him. To begin his argument, he subtly throws doubt on the strength of Laertes' love for his father, Polonius, by placing a series of similarly structured lines in sequence (lines 125 - 127):
Not that I think you did not love your father;
But that I know love is begun by time,
And that I see, in passages of proof,
Time qualifies the spark and fire of it.
Overall, there are many striking incidences of rhetoric used in this act as the characters express private anxieties or seek to influence others.
The links below will lead you to comprehensive information about rhetorical devices.