This Allegro movement, possibly a fragment or lesser-known sonata movement by Mozart, exhibits his characteristic elegance and technical fluency. It likely contains rapid passagework and a well-structured thematic development.
Requires a refined touch and the ability to execute flowing runs with consistent tone quality.
Allegro (K.400) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 7.
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 Allegro (K.400) lasts about 4.5 minutes.
Allegro (K.400) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.