how does swift highlight the human irrationality in the forth part of the voige

I am going to answer if you want to know about his voyage to the land of the Houyhnms. These characters are horses who act in rational ways - they have no use for lies and keep all "instincts" at bay. The Yahoos, on the other hand, are the human characters who are completely uncivilized. They have virtually no redeeming qualities; the appearance of such low characters demonstrate Swift's hatred for mankind in general.