Final answer:
In Act 1 Scene 4 of 'Romeo and Juliet', the primary complication is the hostility between the Capulets and the Montagues, which sets the scene for the challenges Romeo and Juliet's love will encounter.
Step-by-step explanation:
The obstacle set up in Act 1 Scene 4 of Romeo and Juliet is the profound enmity between the two families, the Capulets and the Montagues. This scene occurs right before Romeo and Juliet meet at the Capulet's party. Romeo goes to the party despite being a Montague and against the backdrop of the ongoing feud, setting the stage for the complications their love will face. Adding to the complications, Romeo is initially melancholic over his unrequited love for Rosaline and is persuaded to attend the party in hopes of seeing her, not knowing that his destiny lies with another.