In Part 4 of the book “History of Western Music,” the author explores the music of the 20th century. This period saw a great deal of experimentation and innovation in music, with composers breaking away from traditional forms and structures. Some of the key movements of the 20th century include impressionism, expressionism, serialism, and minimalism. The author also discusses the rise of jazz and popular music, which had a significant impact on the music of the time. Overall, this chapter provides a fascinating look at the diverse and dynamic music of the 20th century.