What is unusual about the finale of Brahms’s Symphony No. 4 in E Minor?

The finale of Johannes Brahms’s Symphony No. 4 in E Minor is considered unusual because it is structured as an extended chaconne—a Baroque form consisting of a set of variations built over a repeated eight-bar bass melody.12

Brahms derived the movement's simple theme from J.S. Bach’s Cantata No. 150 and developed it into 30 highly intricate variations. This choice reflects Brahms's tendency to seek a balance between 19th-century expressive forms and traditional techniques, utilizing contrapuntal craftsmanship that was notably archaic for a symphonic finale of the Romantic era.31