Medtner's cadenzas are essentially independent, complex miniature fantasies that re-imagine Beethoven's themes through a late-Romantic, Russian prism. They are rhythmically dense and harmonically sophisticated.
The performer must navigate thick, multi-layered textures and complex polyrhythms while maintaining momentum.
Comparable difficulty and character — good candidates for what to learn next.
2 Cadenzas to Beethoven's Fourth Piano Concerto 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 Fourth Piano Concerto lasts about 5 minutes.
2 Cadenzas to Beethoven's Fourth Piano Concerto 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 Piano Concerto No.4 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.