It is generally accepted that Johann Sebastian Bach wrote his six Sonatas for Organ for his eldest son, Wilhelm Friedemann Bach, as the not always reliable W.F. often asserted. There is no strong reason to disbelieve it, for they appeared at about the right time and, moreover, are teaching pieces par excellence. There is a world of difference between the familiar Bach organ works in the mold of the various preludes or toccatas and fugues and the Six Sonatas. Compared to those works, these Sonatas are light, transparent in texture, never concerned with display or Baroque flamboyance. They are Trio-Sonatas, works in three voices, irrespective of how many actual players were needed. The voices in these works are independent: one in either hand, the third on the pedals. Ordinarily, each hand plays on its own manual. Thus, the Sonatas test and cultivate the student's physical and mental ability to coordinate all these separate motions of hands and feet, the interpretive ability to project each voice equally and clearly to the audience, and the musical ability to make them meaningful.
All but the sixth sonata are, at least in part, derived from earlier Bach works. The Fourth Sonata is in the form of a concerto for organ. The first movement is a Vivace preceded by a short slow introduction. It is a transcription of a sinfonia of the cantata Die Himmel erzählen die Ehre Gottes, BWV 76 (1723). This number was already a trio for oboe d'amore, viola, and bass line. It is in a ritornello form, with a lively main episode interspersed with contrasting sections. The slow movement seems to have originated as a separate piece. The concluding poco allegro section is vigorous and tests coordination by throwing in triplets that cross hands.
There are various arrangements of this organ sonata as actual trio sonatas, with one or two soloists and keyboard.