"The author's great love of his native country. His master's observations upon the constitution and administration of England, as described by the author, with parallel cases and comparisons. His master's observations upon human nature."
Gulliver starts to hate the Yahoos and love the Houyhnhnms.
In fact, he decides that he never wants to leave Houyhnhnm Land and return to humankind.
The Master Horse gives Gulliver his conclusions: the European Yahoos have only enough reason to make their natural corruption worse.
By clipping our nails, cutting our hair, and generally growing soft, we have also deprived ourselves of the natural protection the Yahoos in Houyhnhnm Land have.
Even though there are outward differences between Gulliver and the Houyhnhnm Land Yahoos, their essential natures are the same: they hate each other more than other animals do, and will fight even without a reason.
The Yahoos of Houyhnhnm Land also love shiny rocks, which none of the Houyhnhnms understand, but which sees to be a trait of the whole human species.
Yahoos are the only animals in Houyhnhnm land who get sick, and they treat each other with medicine made from a mix of pee and poo (urgh).
The Master Horse does admit that European Yahoos have a lot more art than their local Yahoos.
Still, their natures seem essentially identical: for example, Houyhnhnm Land Yahoos also like to choose a leader, usually the weakest and ugliest of the group.
As for women (what the Master Horse calls "she Yahoos" (4.7.15)), he observes that Yahoos are the only ones among animal kind that still have sex even when the woman is pregnant. (Swift's point here seems to be that sex is for procreation, so once a woman's pregnant, she shouldn't need sex – which, if we may editorialize, is kind of icky of him.)
He also notes that Yahoos are unique in having both males and females fighting equally violently with one another.
The Master Horse continues: Yahoos love filth more than most animals.
Also, Yahoos sometimes fall into bad moods or think they are sick for no reason; the only cure for this hypochondria is hard work.
Women Yahoos like to seduce men. Sometimes, if an unknown female comes up to a group of three or four women, those women will clearly judge and then reject her.
Gulliver hears these words and realizes that "lewdness, coquetry, censure, and scandal" (4.7.19) all seem to be instinctive for human women. (For a discussion of Gulliver's views on women, check out our theme on "Gender.")