Act 4 Scene 1 is the brief interaction between Romeo and Mercutio (Benvolio is also present) as they are about to "crash" the Capulet's masquerade ball. Mercutio believes this will provide Romeo the opportunity to find another woman to take his mind off Rosaline.
It is important to note two clues from this scene that indirectly point to the family rivalry. The first is in the opening line by Romeo:
What, shall this speech be spoke for our excuse?
Or shall we on without apology?
He is asking how they are to enter the party, one, without an invitation, and two, as enemies of the host. Essentially, he wants to plan an excuse (or a story) that will get them in unidentified. They are already in costume, but they have not yet come up with any fake identities. Instead, they simply plan to arrive late enough to slip in unnoticed.
Then again at the end of the scene, Romeo tells of a bad dream he had which speaks of something bad that will happen that night:
I fear, too early; for my mind misgives
Some consequence, yet hanging in the stars,
This foreshadowingquote is another indirect example of Romeo's awareness that the feud between his family and the Capulet's will most likely end tragically, and the tragedy will begin that night at the party.