According to the narrator, what four things remain as they were in the period from 1874 to 1890?

You can find the answer to this question in Owen Wister's preface to his novel The Virginian. In this preface, he addresses the reader on the question of what constitutes a historical novel and whether The Virginian is one. According to Wister, although the novel is set at a point not far distant in the past from the time in which Wister is writing, it must be considered a historical novel because it accurately and faithfully portrays a "vanished world" which can no longer be reached outside of the memories of those who experienced it.
Not everything in Wyoming has changed, but according to Wister, the only things which remain unchanged are elements of the natural landscape. The buffalo have departed, and the horsemen, and the sheer wildness of the landscape as it once was. Indeed, the only four elements which are unchanged are the mountains, the sunlight, "the infinite earth," and "the air that seems forever the true fountain of youth."
The type of horseman depicted in this novel, Wister says, has died out since the days the novel depicts, and the Wyoming of the book is a bygone world. The pioneer age, which saw men like the eponymous Virginian fashioning Wyoming into the state it now is, represented an earlier stage in American history. Now, the America of Wall Street has emerged, and an inevitable transition has taken place.