In Much Ado about Nothing, both Beatrice and Benedick challenge gender roles, but it is important to realize that the play has set up a reference for proper gender norms from which Beatrice and Benedick divert. To this end, they are each held up in contrast to two characters who apparently satisfy appropriate behavior for each gender: Hero and Claudio. I will introduce these characters before getting to a discussion of gender in the scene you requested (Act IV, Scene 1).
In Act I, Scene 1, Claudio is lauded for his achievements in battle. He represents youth, vigor, bravery, and nobility. In this war-time play, these are the qualities an audience would expect to see in a masculine hero. The following passage details the description of Claudio.
LEONATO: A victory is twice itself when the achiever brings home full
numbers. I find here that Don Pedro hath bestowed much
honor on a young Florentine called Claudio.
MESSENGER: Much deserved on his part, and equally remembered by
Don Pedro. He hath borne himself beyond the promise of
his age, doing in the figure of a lamb the feats of a lion. He
hath indeed better bettered expectation than you must
expect of me to tell you how.
By contrast, of Benedick it is merely stated that he performed well in the war. He is not lauded for his merits, suggesting that he might fall short of the masculine ideal. Beatrice certainly believes so, as she is often cruel in her jokes and witty banter with him.
Hero is the foil character to Beatrice. Where Beatrice is outspoken and intelligent, Hero is meek and silent. In fact, when she does speak, it is only to keep the peace; for example, her first line of dialogue occurs in Act I Scene 1, where she clarifies that Beatrice is talking of Benedick when she refers to him as Signor Montano in jest. While Hero speaks rarely and only then to ease tension or answer someone who has spoken to her, Beatrice speaks in order to stir up conflict with Benedick or to demonstrate her wit and intelligence.
In Act IV, Scene I, the gendered behavior established by the play is put to the test. Claudio accuses Hero of being unvirtuous. To this society, this means that he believes she is not a virgin. The fallout from these accusations is extreme. Even the mere suggestion that she is not as sweet and innocent as she appears is enough to ruin her reputation. The following passage is the speech Claudio gives accusing her:
There, Leonato, take her back again.
Give not this rotten orange to your friend.
She’s but the sign and semblance of her honor.
Behold how like a maid she blushes here!
Oh, what authority and show of truth
Can cunning sin cover itself withal!
Comes not that blood as modest evidence
To witness simple virtue? Would you not swear,
All you that see her, that she were a maid
By these exterior shows? But she is none.
She knows the heat of a luxurious bed.
Her blush is guiltiness, not modesty.
He is objectifying her by speaking of her like a piece of fruit that has been damaged and attempting to return her to the person who sold her (her father). Marriage is often spoken of in these transactional terms throughout the course of the play.
Benedick diverts from these expectations of marriage by attempting to abstain from the institution altogether, as he states earlier in the play. In this scene, however, he treats love and marriage as something different from a transaction for unspoiled goods. He agrees to trust Beatrice's good opinion of Hero, believing a woman over two well-loved male role models. On little more than Beatrice's good word, he agrees to kill Claudio on her behalf. This depiction of a relationship, though undoubtedly problematic, certainly differs from the one where Hero's thoughts and feelings are inconsequential to Claudio.
In this same scene, Beatrice's reaction to Claudio's accusation also contrasts to the proper gendered behavior that Hero exhibits. When confronted with the suggestion of her "crimes," Hero faints, as women are often portrayed as doing during the time period. Beatrice, on the other hand, goes on a rant about how horrible Claudio is, saying that she would kill him if she were only a man. Even voicing these opinions defies gender norms and demonstrates a vastly different way of responding to the same event.
In a play where marriage is the norm and cuckoldry is a man's biggest worry, both Benedick and Beatrice subvert stereotypes by their behaviors, as well as by their approach to love and marriage. Act IV Scene I sets up masculine and feminine expectations through Claudio and Hero, and it also shows how both Benedick and Beatrice divert from these norms.