How does the narrator of "The Raven" change in the fourth stanza?

In the fourth stanza, the narrator undergoes a bit of a change in courage. In the third stanza, he says, he is filled with "fantastic terrors," likely referring to his fears about who could be knocking at his door at midnight. His heart is beating fast, and he "stood repeating" the same reassuring phase about it only being some visitor (as opposed to something scarier).

In the fourth stanza, as he rises from his chair, he calls to whoever is at the door, explaining why it has taken him so long to answer their knock. However, in the last two lines of the stanza, when he does open the door, he sees "Darkness there and nothing more" (line 24). With this particular phrase, then, the potential for supernatural happenings in this poem is established as a possibility. Prior to this, the narrator has attempted to be practical, but at this point, into the fifth stanza, he begins to wonder if it isn't the ghost of his lost love, Lenore, who has come to call. The fact that there was no one there to knock at his door puts him in the right frame of mind to receive the raven a few stanzas later and to attribute to that raven so many different supernatural possibilities.