The fourth volume of Masaaki Suzuki's Bach works for organ series features one of the most important surviving instruments of Bach's time, made by the German organ builder Christoph Treutmann the Elder. Widely known for it's extraordinary tonal quality, the instrument was built between 1734 and 1737. A recent general restoration preserved all essential structural elements or renewed them, remaining faithful to the originals, making it an ideal instrument for Bach interpreters who wish to come close to the sound ideas of the Leipzig Thomaskantor. Suzuki now takes up the Orgel-Büchlein (literally, 'little organ book'), a collection of 45 short chorale preludes on melodies from the Lutheran hymn book, a project that came into being in connection with Bach's appointment as organist and chamber musician at the Duke's court in Weimar in 1708. Presenting chorales for different periods of the church year, this collection serves as a general guide to text-based composition focusing on specific word-sound relationships and content-specific musical expression. Two Preludes and Fugues complete the first volume dedicated to the Orgel-Büchlein, illustrating the principle of variety and structure historically practised by concert organists in order to demonstrate the tone colours and expressive possibilities of their instrument.