I think it is an oversimplification of Swift’s work to simply say that Yahoos are humans. True, there are characteristics humans and Yahoos share, but one may also argue that humans have things in common with Houyhnhnms Yahoos, like people, are very fearful and defensive (pg. 2419-2420). Gulliver uses violence against the first Yahoo who approaches him, so it is understandable that the creature would react with anger, and others would try and attack Gulliver for hurting one of its own kind. This mentality is common not only to Yahoos and humans, but all creatures have a sort of need to protect their own kind from foreign danger.
But I also think that comparisons can be made between the Houyhnhnms and humans. They have a form of social order, unlike the Yahoos. While this social order seems to be communist or socialist in nature, it can still be compared to social orders for humans. Another, possibly negative way that Houyhnhnms are like humans are the fact that they are not accepting of others. Gulliver tries to incorporate himself into their world, but they continue to view him as different and an outsider of the group. But this mentality was also seen in the defensiveness of the Yahoos. Almost all creature groups have a fear of what is different.
The role of sex in both the world of Yahoos and Houyhnhnms have parallels to human reproduction. Swift makes young Yahoo women impulsively lustful (pg. 2444), and unable to control their urges. Though this make be an extreme version of the case, human are known to be impulsively lustful and submissive to sexual and physical urges. For Houyhnhnms their procreation is very technical and clinical. This mentality also exists in human culture, and was a major part of society in Swift’s times. Arranged marriages for procreation within the same social classes was prevalent, and their was a formality to sex in this regard. Especially thinking about royal families at the time in which the need to create the appropriate offspring led to intra-family relationships.
I believe that to say Yahoos are humans is too easy, and ignores the similarities that humans also share with Houyhnhnms, both good and bad. I think the two species groups of Book 4 represent two ends of a spectrum that has human behavior across it.