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.