Possible Clues:

• Key with four flats
• Key of Beethoven's "Appassionata" Sonata
• Key of Chopin's Piano Concerto No. 2
• Relative of A-flat major
• Key of Brahms's Piano Sonata No. 3
• Beethoven's 'Appassionata' is in it
• Key of Chopin's 'Piano Concerto No. 2'