Johann Ludwig Krebs was a favorite student of J.S. Bach, and this piece demonstrates his mastery of the fugal and choral styles. It is musically distinctive for its expressive chromaticism and deep, vocal-like melodic writing.
At Henle 4 this is an intermediate piece that expects secure technique and real attention to phrasing. The main demands: independent voices that must each sing on their own line; ornaments that need to sound effortless, not squeezed in; and chord voicing — bringing out one note inside a handful of others.
Beyond that, expect a true singing tone, with the melody floating above the accompaniment.
Mastery of Baroque ornamentation and the ability to project the melody clearly through the polyphonic texture is essential.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Where this piece's skills lead: slightly harder works making similar technical demands.
Chorale Prelude - Ach Herr mich armen Sünder is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Chorale Prelude - Ach Herr mich armen Sünder lasts about 5 minutes.
Chorale Prelude - Ach Herr mich armen Sünder is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Berceuse Triste by Alfredo Casella, Berceuse Triste by Alfredo Casella, Bonheur perdu by Paul Barbot — see the full similar-pieces list on this page.