The fourth voyage of Swift's "Gulliver's Travels" is one of the most studied voyages. It is a veritable happy hunting ground for critics. Critics have concerned with two problems, "what do Yahoos and Houyhnhnms stand for? And what moral Swift was drawing from them?” Allen Bloom thinks that it is a projection of platonic utopia but it is a simple satire against humanity. Swift has More’s utopia in this voyage.
The fourth voyage takes Gulliver to land of Houyhnhnms and Yahoos. There are bestial humans and rational horses. Houyhnhnms represent the "horses" and "perfection of nature". They are beautiful horses, strong, healthy, and simple. They lived in beautiful houses. They have an organized unity. There is no vice among them. There are no quarrels, no lie, no theft, no dishonesty and no artificiality. Marriage to them is not source of emotional and sensual pleasure. It is only for them to continue reproduction of race. They can change their wives and children if they require. They cannot allow any stranger to enter into their land except any species called "Yahoos".
Yahoos are pure sensual animals. They are kept away from Houyhnhnms. They love and store gold without knowing its value. Sometimes they spent time in fighting with each other. They also bite each other. Their females attack their males and their males attack females in order to fulfil sensual pleasures. Their bodies look ugly and repulsive smell comes out of them. In short, they have no reason.
Gulliver himself finds in a dilemma. He is in strange situation and does not know what to do. He physically is Yahoo and mentally he justifies himself with Houyhnhnms.
Swift has cut sharply human nature in two parts--- rational and sensual. Yahoos lack reason and Houyhnhnms lack sympathy and aesthetics. Both are incomplete and so dangerous. If man is rational then he is equally dangerous. So Gulliver is divided in Yahoos and Houyhnhnms.