Schubert's Op. 142 Impromptus are masterpieces of lyricism and harmonic invention, with this A-flat major piece standing out for its beautiful, song-like melody and serene atmosphere.
Developing a rich, singing tone and seamless legato phrasing is crucial for this piece.
4 Impromptus (Op.142) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 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 Impromptus (Op.142) lasts about 8 minutes.
4 Impromptus (Op.142) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.