(Ok, this is very late. Sorry about that. The past two months have been so hectic that I've had trouble keeping on schedule.)
The fourth and final part of 'Gulliver's Travels' recounts his visit to the land of the Houyhnhnms, which from now on I will refer to as 'H.'. This time he isn't shipwrecked but cast off the ship by some pirates. He washes up on an unknown land and encounters some savage primate beasts. He is rescued by some horses and we soon learn what the deal is with this voyage.
The land of the H. is one where the relationship between man and horse has been reversed. The horses are sentient and in command. The men are savage in the truest sense, in a way that we would regard as sub-human. They are called Yahoos.
Gulliver is adopted by one of the H. and after some time he learns their language. Swift tells us that the H. are noble and just. In fact, they're so noble and just that Gulliver soon becomes ashamed that he is merely a man from a human civilization. He's also horrified by the comparison to himself and the Yahoos.
In time, Gulliver sadly tells the H. just how badly the horses back in Europe are treated. When he tells them of the process of gelding them, it's enough to make you wince and fear for Gulliver's safety. He is soon spirited out of the country though by his master/owner. He returns to England and finds the presence of other people bothersome. Instead, he prefers the company of his own horses.
This section is weaker than the first three. The H. are put on a high pedestal that frankly sounds undeserved. In fact, this section makes reflection on the previous voyages a little sad, especially that of Brobdingnag. They too were nearly too good to be true. Did they deserve that, or was that also a simple authorial trick?
Swift's contemporaries thought that the last section of Gulliver was proof that he was slipping mentally. I don't care for that kind of explanation, because it allows the reader to simply dismiss arguments instead of meeting them head on. But I think it's obvious that the fourth voyage isn't up to the standard of the first three.
There are some interesting arguments to be made regarding the treatment of animals. I don't know how much Swift had to do with popularizing such arguments but it wouldn't surprise me to find out that it was quite a bit. We treat horses much differently than we do men, because, well, there are big differences. We recognize men as sentient in a way that horses clearly aren't. If we reverse those roles, then of course the men (Yahoos) would be treated differently. And quite rightly.