Chapter four of the book “History of Western Music” by Donald Jay Grout covers the period of music from the Baroque era to the Classical era. During this time, music became more structured and focused on melody and harmony. Composers such as Bach, Handel, and Mozart created masterpieces that are still celebrated today. The chapter also discusses the development of opera and the rise of instrumental music. Overall, this period of music was marked by a shift towards more refined and elegant compositions that continue to influence music today.