This present volume is an updated edition of Series IV, Volume 6, of the New Bach Edition (NBA), prepared by Dietrich Kilian in 1964. The discovery of new sources and a reassessment of existing handwritten copies have brought about changes to the contents of the volume, which now includes the Fantasia in C minor (BWV 1121), the Fantasia in G major (BWV 571), the Fugue in G major (BWV 577) and the C major version of the Toccata and Fugue in E major (BWV 566). On the other hand, the Prelude in G major (BWV 568) as well as the alternative versions of the Fugue in C minor (BWV 574a) and the Prelude in A major (BWV 536a) have been discarded as presumably spurious. In collaboration with the Bach Archive Leipzig and its Bach digital database, links are provided to all the major sources. The website http://links.baerenreiter.com enables readers to access the relevant page in the source simply and conveniently via short URLs. The links stored there are examined for their validity on a regular basis.