"The author's economy, and happy life, among the Houyhnhnms. His great improvement in virtue by conversing with them. Their conversations. The author has notice given him by his master, that he must depart from the country. He falls into a swoon for grief; but submits. He contrives and finishes a canoe by the help of a fellow-servant, and puts to sea at a venture."
Gulliver is absolutely content: he has all the shelter (thanks, in part, to the building skills of the sorrel nag), clothing, and clothes he needs, and he feels completely calm and at peace.
Gulliver has lots of nice friends among the Houyhnhnms; in fact, he's proud that the Houyhnhnms sometimes say that he "trots like a horse" (4.10.4).
Sadly, one morning the Master Horse comes to see Gulliver and to tell him that the Houyhnhnms have voted that Gulliver must go away. They worry that such a smart Yahoo might encourage the other Yahoos to rise up and kill the Houyhnhnm's cattle.
The Master Horse tells Gulliver that he will be sorry to see him go – but he will have to.
Gulliver is heartbroken at this news, so much so that he actual faints.
The Master Horse gives Gulliver two months to finish his boat, which he builds with the help of the sorrel nag.
Gulliver explores the coast with his telescope and finds a small island about three and a half miles away that he can reach in his boat.
Finally, when the day comes for Gulliver to leave, the Master Horse and his whole family come to see him off.
Gulliver cries and kisses the hoof of the Master Horse.