"Opus 4" is a composition by the contemporary composer Karlheinz Stockhausen. It is part of his larger work, Kreuzspiel, which explores the relationship between sound and space. The piece is known for its innovative use of rhythm and texture, showcasing Stockhausen's unique approach to music. The composition is typically performed by a small ensemble, allowing for intricate interactions between the musicians. "Opus 4" emphasizes the importance of improvisation and spontaneity, reflecting Stockhausen's belief in the transformative power of music. This work is often studied for its contributions to modern classical music and its influence on later composers.