What are some examples of foreshadowing in Romeo and Juliet act 1 scene 4?

If you are asking what "foreshadowing" is, it's a literary technique where the author or playwright gives hints or clues about what will happen later in the story. If you have read Romeo and Juliet (and I hope you did, even if the language is difficult-- a lot of people have quoted from it or referred to it over the years... including pop stars like Taylor Swift; it will be helpful to you later on if you recognize some of those quotes and allusions), you know there are clues about what will happen to Romeo as well as what will happen to Juliet.

In Act 1, scene 4, Romeo and two of his friends are trying to get into a costume party held at Juliet's residence; although Romeo is masked, he is still worried that he will be discovered as a Montague. In fact, it's a dangerous thing for him to even try to attend the Capulets' party, since the Capulets and the Montagues are sworn enemies. He and his friends are joking around (and some of the jokes have two possible meanings, one of which is about sex... and yes, back then, they did joke about sex, just like today); but Romeo admits he is worried about attending the party: he says he had a dream that it was a bad idea to go.

But the real foreshadowing occurs next, after what seems like a bizarre speech from Romeo's friend Mercutio, about "faeries" (winged spirit-creatures often mentioned in myths). It's kind of an angry speech, a bitter recitation of how Queen Mab appears to various people (he gives examples) in their dreams and tempts them and gives them what turns out to be false hope. Romeo listens for a while and then says he has worries of his own: "Some consequence yet hanging in the stars/ Shall bitterly begin his fearful date/ With this night's revels, and expire the term/ Of a despis├Ęd life closed in my breast/ By some vile forfeit of untimely death." In other words, he fears that tonight's party will be the beginning of something bad, something that might even lead to his death. There are other examples in the scene too, but having gotten you started, I hope you can re-read it and get more out of it.