How does swift highlight the human irrationality in the fourth part of the voyage?

Gulliver's Travels is a multilayered text that can be interpreted in multiple ways. One can look at the Yahoos' description as an example of Swift's misanthropy. By the end of his stay with the nearly utopic Houyhnhm society, Gulliver was so disillusioned with the values and customs of his own race that he started to regard all humans as Yahoos. He got emotionally deranged when he was separated from his Houyhnhnm friend. Gulliver's irrational urge to never leave the Houyhnhnms stemmed from complete lack of reason. There is also a possibility that Swift wanted to portray the dark side to humanity through Yahoos. 

Note: This is one point of view. There may be other opinions which can be supported by examples from the text.