There are four scene fours in the play. You must state which Act you mean. However since the only scene four that takes place before Capulet's feast is Act 1 Scene 4, I'll assume that is what you mean. In this scene he says, "my mind misgives Some consequence yet hanging in the stars shall bitterly begin his fearful date With this night's revels". As a somewhat different excuse, he says "I am not for this ambling, Being but heavy", and "I have a soul of lead" and "I am too sore empierced with [Cupid's] shaft to soar with his light feathers", all of which appears to refer to his conversation with Benvolio in Act 1 Scene 1
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In Act I Scene 4 he complains of "heaviness", which means feeling depressed. He gets talked into going to the party anyway.
Romeo gives the reason that he has a premonition or feeling of impending doom if he attends the party. He believes that something bad may happen if he goes, and therefore chooses to stay away.