In Scene 2 of Act III, Benedick lacks wit and reveals the symptoms of a lover. Scene 4 parallels that scene, for it reveals how Beatrice is reacting to what she had overheard in the garden and how she now feels about Benedick.
Beatrice does have a brief witty exchange with Margaret (lines 37-42). Although Beatrice has not lost all of her wit, the reader should note that Margaret makes the last witty remark at that point. Beatrice ignores her and then speaks plainly to Hero. Thus, in this witty battle, Margaret is actually the winner.
A few lines later Beatrice actually does appear to have lost all of her wit:
MARGARET: Well, an you not be turned Turk, there's no more sailing by the star.
BEATRICE: What means the fool, trow? (47-49)
Margaret's line suggests that Beatrice has completely reversed her feelings about love: Beatrice is no longer a sworn bachelorette. The word an means if, and the word Turk is used generically for an enemy of Christianity. Thus, Margaret implies that Beatrice has switched sides or turned traitor. Earlier Beatrice was against love and marriage, but now she is for love and marriage. The word star refers to the North Star, the one constant or unmoving feature in the night sky which helps sailors find their direction at night. Essentially, Margaret is stating in a witty way that if Beatrice is not in love, then the North Star is no longer constant. In other words, there is a change, but it is not in the North Star, it is in Beatrice. Earlier Beatrice would have had no trouble understanding Margaret's joke. But now that Beatrice is in love, her wit and understanding have left her (the word trow means something like "I would like to know" or "I wonder").
If the audience does not quickly perceive the change in Beatrice with these lines, Shakespeare indicates the same idea a few lines later in a plain and direct way. Beatrice asks Margaret how long has she been acting as a wit ("professed apprehension" in line 57). Margaret responds with the following:
Ever since you left it. (58)
Margaret thus declares that Beatrice no longer is so witty. The scene ends with Margaret and Hero making a few more jokes about Beatrice being in love. Margaret and Hero tease Beatrice just as Don Pedro and Claudiuo had teased Benedick in the earlier scene.