This is a duplicate reference to Medtner's highly individualistic cadenza for the Fourth Piano Concerto. It is marked by its idiosyncratic harmonic progressions and extreme pianistic demands.
The primary hurdle is the integration of complex, layered textures that require advanced hand independence.
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 2 Cadenzas to Beethoven's Fourth Piano Concerto by Nikolai Karlovich Medtner, Ballade No.1 by Frederic Chopin, Ballade No.1 by Frédéric Chopin — see the full similar-pieces list on this page.