What happens in Act 1, Scene 4 of Shakespeare's Romeo and Juliet, the masquerade party at Capulet's house?
I believe you meant to ask about Act 1, Scene 5, the scene in which the masquerade ball takes place in Capulet's home. The scene starts out very lightly with the servants having a comic exchange and Capulet encouraging every lady to dance. He very wittily exclaims that if a lady doesn't dance, he'll assume she has corns, as we see in his amusing lines, "She that makes dainty, / She, I'll swear, hath corns" (I.v.18-19).
The scene becomes a bit darker when Tybalt hears Romeo's voice, recognizes him, and starts to charge at him with his sword for trespassing. But his uncle stops him saying that Romeo isn't doing any harm, making Tybalt feel insulted now by both his uncle and Romeo, which leads to Tybalt's fateful decision to challenge Romeo to a duel later in the play.
Finally, the mood of the scene becomes a bit more intense when the two lovers meet, exchanging a repartee containing a great deal of religious motifs and imagery. Due to the religious motifs, the repartee is both witty and serious. The seriousness of their exchange enhances the seriousness of the next moments in which they learn who they both really are--mortal enemies. Juliet phrases it best when she declares, "My only love, sprung from my only hate!" (147).
Hence, if one were to analyze this scene, one would see that it starts out very light and gay but then grows more serious as it progresses.