How does Shakespeare present the changes in Benedick and Beatrice’s relationship between 1:1 and 4:1 in the play 'Much Ado About Nothing'?

In the very first scene of Much Ado About Nothing, Beatrice asks about Benedick, making fun of him to a messenger:

MESSENGER: I see, lady, the gentleman is not in your books.

BEATRICE: No; an he were, I would burn my study.

Before they even meet, it is established that the two love to argue with each other. When Benedick arrives, he and Beatrice quickly spar:

BEATRICE: I wonder that you will still be...

In the very first scene of Much Ado About Nothing, Beatrice asks about Benedick, making fun of him to a messenger:

MESSENGER: I see, lady, the gentleman is not in your books.

BEATRICE: No; an he were, I would burn my study.

Before they even meet, it is established that the two love to argue with each other. When Benedick arrives, he and Beatrice quickly spar:

BEATRICE: I wonder that you will still be talking, Signior Benedick: nobody marks you.

BENEDICK: What, my dear Lady Disdain! are you yet living?

It is established that they have known one another for a while, and Benedick does assert that Beatrice is beautiful, “were [she] not possessed with a fury.” They continue to mock each other until Benedick becomes offended and declares, “I would not marry her, though she were endowed with all that Adam bad left him before he transgressed.”

Benedick’s friends determine to matchmake the quarrelsome pair. The men pretend that Beatrice loves Benedick, and the women similarly fool Beatrice. Benedick immediately decides to “be horribly in love with her.” Beatrice also declares, “Benedick, love on; I will requite thee.” Much to the amusement of their friends, the potential couple become lovesick.

The first scene of act four features Hero and Claudio’s disastrous wedding. Claudio accuses Hero of liaising with a man the night before. Benedick is the only one of his friends who sides with Hero, likely because of his love for Beatrice. The two finally confess their love for one another. Benedick admits, “I do love nothing in the world so well as you.” Beatrice concedes, “I love you with so much of my heart that none is left to protest.”

In contrast to their past behavior, the two are humble and vulnerable together. These former enemies have become lovers. Their relationship does not lose its fire, however. Beatrice demands that Benedick “kill Claudio,” his close friend. He eventually agrees.