It could be seen as a marker for a specific point in the novel. Maybe the author uses 'day four' to build up to a climax that occurs on that day. It might also signify a turning point in the story, where the plot takes a different direction compared to the previous days.
One way to interpret it is that it refers to the fourth day within the narrative of the novel. It might be a day that stands out in some way, like a day full of action or important revelations.
Interpretation of 'day four a novel' can vary. It could potentially be the fourth day of a character's journey in the novel, whether it's a physical journey like traveling to a new place or an emotional journey such as coming to terms with a difficult situation. It may also be a way for the author to break up the story into time - based segments, with this being the fourth segment, and something special or unique is supposed to happen on this day within the world of the novel.