Early on the wedding morning, Hero and her gentlewomen are preparing Hero for her wedding. Hero sends Ursula to bring Beatrice. Margaret is critical of what Hero has chosen to wear but quickly backs off when Hero scolds her lightly. Margaret also teases Hero about Claudio and his soon-to-be presence in her bed.
When Beatrice arrives, she says that she is not feeling well. Margaret teases her that she is sick with love. Ursula returns to tell them that the men have come to take Hero to the church.
Notice the parallels between Beatrice's illness, whatever it is, and Benedick's "tooth-ache" in Act III, Scene 2, and between the teasing of Beatrice by Hero and Margaret here and the teasing of Benedick by his friends in the earlier scene. Both Beatrice and Benedick are suffering through this important change in their mindsets.
Remember that Margaret has participated in the love scene in Hero's window the night before, dressed in Hero's clothes. Perhaps this makes her edgy, so that she argues a bit with Hero about the choice of wedding clothes. Her conversation with Hero has sexual connotations (the quips about the weight of a man), and her conversation with Beatrice has uncharacteristic bite to it as she teases Beatrice about Benedick. Her words and actions suggest that she is unaware that her encounter with Borachio was designed to destroy Hero's reputation — even unaware that they were observed.
Carduus benedictus was the Latin name for Holy Thistle, a popular general remedy of the day, especially for problems associated with the heart. As Margaret intended, the Latin name reminds Beatrice about Benedick: "Benedictus, why benedictus? You have some moral in this benedictus." Though Margaret denies the connection, she goes on to talk about Beatrice being in love. This line is a good example of Shakespeare's use of language and ideas of the time, which is likely to be partly missed by today's readers or playgoers unfamiliar with carduus benedictus and its use.