In The Decameron, what is the overall theme of day four?

The overall theme of Day Four, under Filostrato, is love stories with unhappy endings. The idea is that love often results in death and sorrow. Before these stories begin, Boccaccio defends his stories, including his inclusion of women in the stories, stating: "Others, making a show of wishing to speak more maturely, have said that it sorteth ill with mine age henceforth to follow after things of this kind, to wit, to discourse of women or to study to please them." In other words, people have criticized Boccaccio for telling stories about women at his advanced age. He tells the tale of Filippo Balducci, whose son does not meet women until age 18 and then becomes obsessed with the opposite gender. This leads Boccaccio to conclude that "nature was stronger than his wit." In other words, nothing can prevent men from loving women. 
One of the most famous stories in The Decameron is Lisabetta's story, the fifth tale on day four. In this tale, Lisabetta's brothers kill her lover, Lorenzo. The slain lover then appears to Lisabetta in a dream dressed in rotting clothing and tells her where he is buried. She disinters him and cuts off his head and wraps it in a cloth and then buries it. She plants basil in a pot over her lover's head, and she weeps over it every day. Her brothers remove the pot from her, and she dies as a result. According to experts (see the link below), the references to the decomposition of Lorenzo's body are an allusion to the plague surrounding Florence at the time. 
https://www.brown.edu/Departments/Italian_Studies/dweb/themes_motifs/plague/lisbetta.php