Bach's '4 Duettos' are highly contrapuntal works that showcase his mastery of imitative writing for keyboard instruments. These pieces are intellectually rigorous, demanding clarity of line and precise articulation.
Developing independent finger control and the ability to delineate multiple melodic lines simultaneously is paramount.
4 Duettos is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 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 4 Duettos lasts about 15 minutes.
4 Duettos is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.