in the plot descriptions, Gulliver’s ship is blown off course just north of Madagascar. The storm pushes the ship “about five hundred leagues to the east,” where the crew first sights the land of Brobdingnag. The contradiction occurs because based on the ship’s location, they would be in the Oceania region, nowhere near North America. Furthermore, Brobdingnag is described as the size of a continent, which does not match the map provided.

In this instance, Swift is satirizing the unreliability of travel writings and geographic descriptions.

Another example of satire is the culture of Brobdingnag. Swift describes it as a particularly advanced culture that enjoys poetry, mathematics, and ethics. However, due to the massive size of the region’s inhabitants, Gulliver is forced to use his hanger to fend off vermin. This satire shines a light on the relativity of advanced civilization. For the giants, they exist in an advanced civilization, but for Gulliver, the civilization and lifestyle is anything but.

As F. P. Lock observes,

Swift's original impulse in writing Gulliver's Travels was certainly to create a general satire on the follies of European civilization as a whole. . . . (F. P. Lock, The Politics of Gulliver's Travels.  Oxford: Oxford University Press, 1980. p. 69)

In Part I, Swift uses the Emperor of Lilliput, whose mind is as limited as his body is small, to satirize the greed, corruption, and war-mongering of England's King George I and Queen Anne.  In Part II, the satire rests on the contrast between the Brobdingnagian king, who is the essence of a benign and moral leader, and George I, who looks even worse by this contrast than the Emperor of Lilliput.  Perhaps Swift's most biting element of satire in Part II lies in the interchange between Gulliver and the king about the use of gunpowder and cannons as a tool of political power.

Gulliver introduces the King to one of the most powerful tools of warfare, which has the additional benefit of enabling a king to control his own people:

I told him of an invention, discovered between three and four hundred years ago, to make a certain powder, into a heap of which, the smallest spark of fire falling, would kindle the whole in a moment, although it were as big as a mountain, and make it all fly up in the air together, with a noise and agitation greater than thunder.

As one would expect from a morally just leader, the King is not horrified by the concept of such a weapon but is also surprised that such small creatures (Europeans) would harbor such horrendous thoughts, especially without any apparent thoughts of remorse about the terror and bloodshed of such weapons.  He is, in short, utterly mystified that Gulliver's fellow Europeans could regard such destructive power without any moral reservations.

Gulliver's astonishment highlights Swift's condemnation of European savagery and its callous disregard of human rights:

A strange effect of narrow principles and views! that a prince . . . of strong parts, great wisdom, and profound learning . . . should, from a nice, unnecessary scruple, whereof in Europe we can have no conception, let slip an opportunity put into his hands that would have made him absolute master of the lives, the liberties, and the fortunes of his people! 

By setting up the King of Brobdingnag as the fool who fails to recognize the powerful tool Gulliver is willing to put into his hands, Swift creates the dramatic contrast between the just ruler of this exotic land and the current King of England who, by implication, would embrace such a weapon in a heartbeat.  To characterize the Brobdingnagian response as the "effect of narrow principles and view" points up the perversity of Gulliver's and, by reference, the European attitude toward such power.

Swift, through the voice of Gulliver, then, has managed to condemn the  European ruler's lust for power and acceptance of mass destruction by merely creating its opposite, a humane, morally just leader who is shocked that anyone could think the destructive power represented by gunpowder could possibly be a beneficial thing.

One good example of satire in Brobdingnag is the status of laws. Instead of the overwritten, obscure laws of most governments, Brobdingnagian laws are limited and plainly written, with only one possible interpretation:

No law in that country must exceed in words the number of letters in their alphabet, which consists only of two and twenty... They are expressed in the most plain and simple terms, wherein those people are not mercurial enough to discover above one interpretation: and to write a comment upon any law, is a capital crime.
(Swift, Gulliver's Travels, eNotes eText)

By limiting the laws to a short length and one interpretation, there is no structure for a court and lawyers to tie up public funds and time with long legal battles. This removes the possibility of corruption, since no one can be bribed or threatened to interpret the laws any other way. Also, the simplicity of the laws means that anyone can understand and apply them, instead of limiting the legal audience to those verse in dense legalese. Here, Swift satirizes the law structure of England and the United States, where laws are long and difficult to understand in their language and interpretation.

The main satirical point in part 4 of Gulliver’s Travels is essentially the same as that in the first three books, though it is perhaps even more bluntly expressed. That point is to ridicule and deprecate humans by pointing out our foolish and despicable behavior. In part 4, this is achieved by making Swift’s ideal beings (the wise and virtuous Houyhnhnms) resemble horses, while the Yahoos (the filthy, degenerate creatures they regard as fit only for slavery) look almost exactly like people.

Gulliver reports that the Houyhnhnms initially mistake him for a Yahoo. He differs from one in a few small matters: his smooth white skin, the hairlessness of most of his body, and his habit of walking upright. Otherwise, he resembles the creature perfectly. The method of satire is very similar to that of part 1, in which it was the diminutive size of the Lilliputians which exposed them to the satirist’s ridicule. In part 4, the brutish nature of the Yahoos is used to ridicule the pride of human beings. When Gulliver returns to England at the end of part 4, he says that he finds it difficult to be reconciled to the company of Yahoos (by which he means all the people he encounters, including his wife). Above all, he cannot understand how such a degraded creature can be proud of itself:

My reconcilement to the Yahoo kind in general might not be so difficult, if they would be content with those vices and follies only which nature has entitled them to...but when I behold a lump of deformity and diseases, both in body and mind, smitten with pride, it immediately breaks all the measures of my patience; neither shall I be ever able to comprehend how such an animal, and such a vice, could tally together.

The aim of Swift’s satire here is, as ever, to mortify the pride of humanity.

Swift satirizes the assumption that humans are more intelligent and rational than other species when Gulliver meets up with the Houyhnhnms on an island south of Australia. The Houyhnhnms are horses, but they have devised a society far more orderly, sensible, and peaceful than that of European humans. In this world, the humans are savages called Yahoos, and the horses run society. Swift thus satirizes or pokes fun at European notions that European humans are innately superior to all other beings. This has been understood as both an early attempt at defending animal rights, and, more commonly, as an attack on the dismissive treatment of non-European populations by the Europeans.

This section, however, also satirizes the Houyhnhnms. The Houyhnhnms are depicted as too rational and too lacking in emotion. This satirizes rationalism—we might think, in parallel, of the narrator of Swift's "A Modest Proposal," who values human life less than a cost-effective solution to the problem of poverty. The Houyhnhnms don't seem to place much value on individual life: they don't grieve the death of close relations, for example, and they will trade off their children with one another to achieve the ideal family of one girl and one boy without seeming to feel any pain at losing their own child.

Swift also satirizes Gulliver's extremism when he returns to European civilization. Gulliver misses the point of what the Houyhnhnms have to offer when he spends his time in a barn trying to talk to the horses and rejecting human contact. He mistakes the outward form of the Houyhnhnms for their inward intelligence. He misses the good that exists in people like Don Pedro, and now sees all humans as Yahoos. A wiser person might glean the wisdom of Houyhnhnms' way of life and try to bring it back to human society. He might realize that all cultures have strengths and weaknesses.

I would suggest that the fourth part of Gulliver's Travels contains a criticism of the rationalism the Enlightenment espoused. Indeed, it's worth keeping in mind that Jonathan Swift himself could be quite critical of the Enlightenment and its focus on reason, as can be seen in a work such as "A Modest Proposal." That particular pamphlet seemed to suggest that reason on its own (unsupported by empathy and genuine human emotion) can lead one down paths that are utterly monstrous. I'd suggest this same perspective seems to be at work in the episode of the Houyhnhnms.

As has already been expressed, much of the satire of this fourth book depends on the reversal between the humanoid yahoos (who are treated within the narrative as beasts) and the hyper-rational Houyhnhnms. However, while Gulliver certainly idolizes the Houyhnhnms, that extreme rationalism contains an emotional coldness that turns into cruelty. Thus, we can observe in chapter 9 these hyper-rational horses holding a debate concerning the proposed extermination of the Yahoos.

Later, in chapter 10, Gulliver himself faces rejection by the Houyhnhnms. Readers of Gulliver's Travels must often differentiate between Gulliver's opinions and Swift's own opinions, which can often conflict with one another. I'd suggest that such a tension may well be in effect with Gulliver's time among the Houyhnhnms.

In Part 4, Gulliver finds himself in the land of the Houyhnhnms, who are intelligent and moral horses. He at first believes the horses are owned by humans, but then he realizes that the horses are their own masters. On the other hand, the Yahoos, who are human-like characters, live like animals and are nasty brutes.

In many ways, the satire is a very simple one in this case.  The Yahoos are meant to represent the general condition of the human race.  As Gulliver comes to understand, these beings are so incredibly selfish and so driven to vice and disgusting behavior, that any time they gain an advantage or find a way to get more resources, etc., they immediately turn that windfall or that gain into a further journey into vice.

By contrast, he makes it clear that the Houyhnhms are so incredibly rational.  So much of what they do makes great sense, etc.  Of course Gulliver tries as hard as he can to make himself into the latter and avoid interaction with or implication that he is in fact the former.

This satirical treatment of the human race is only strengthened by later parts of the story when Gulliver returns home and voices his disgust with the behavior of the people he interacts with.

In book four, Gulliver meets the Houyhnhmns, a race of very rational horses, as well as the Yahoos, humanoid animals that satirize the very worst aspects of the human race.  The Yahoos are personally foul, unclothed and uncouth, defecating on Gulliver from trees when they first see him.  They violently fight all the time, over insignificant disputes or items.  They are greedy and covetous, doing whatever they can to acquire, even killing one another for some small, meaningless object because it is shiny.  Even when they have plenty to share, more than they can possibly need, they still selfishly fight over it and take as much as they can.  The females are also sexually licentious, and one even tries to force herself onto Gulliver when she finds him bathing.  In short, they are morally reprehensible and disgusting in every way, and they are meant to point out our own very worst faults: our greed and selfishness, as well as our violence and lack of concern for one another.