Schubert's Op. 90 Impromptus are cornerstones of the Romantic piano repertoire, with this C minor piece being a virtuosic showstopper characterized by its driving energy and bold dynamic shifts.
Mastering the rapid octaves and challenging leaps requires significant hand strength and accuracy.
4 Impromptus (Op.90) 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 4 Impromptus (Op.90) lasts about 6 minutes.
4 Impromptus (Op.90) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.