Given that Mary Shelley's novel Frankenstein is a Romantic one, the inclusion of nature is necessary. That said, the end of chapter four offers (and personifies) nature as being in possession of irony.
In order to define the irony of nature, one must examine nature's place in the novel. Essentially, Victor has "sinned" against nature by taking the power of giving life out of the hands of women and placing it in the hands of a man.
While nature is, ultimately, about regeneration, the death of nature in this chapter is ironic.
The leaves of that year had withered before my work drew near to a close.
This quote shows the irony behind Victor's desire to create life at the same time nature is ending it. This also offers the reader foreshadowing of what is to come. While Victor's desire to create life comes from a heart-warming place (the death of his mother), it supersedes the order of natural things. Given that Victor has now changed the face of nature as the world knows it, nature is reacting against his abomination against nature.