Feinberg's cadenza is characterized by its technical bravura and dense, almost Rachmaninoff-esque harmonic language. It demands high agility and a powerful command of the piano's full register.
The main challenge is maintaining clarity during rapid, high-velocity runs and dense octave passages.
Comparable difficulty and character — good candidates for what to learn next.
2 Cadenzas to Beethoven's Piano Concerto No.4 is rated Henle level 8 of 9 — concert repertoire, roughly diploma level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 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 2 Cadenzas to Beethoven's Piano Concerto No.4 lasts about 5 minutes.
2 Cadenzas to Beethoven's Piano Concerto No.4 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 Rhapsody in Blue (Solo Piano) by George Gershwin, Hungarian Rhapsody No. 2 in C-sharp minor by Franz Liszt, Fantaisie-Impromptu in C-sharp minor by Frédéric Chopin — see the full similar-pieces list on this page.