Noctave
The piece
Like the third, this fourth Impromptu D. 899 only appeared in 1857, published by Carl Haslinger. Despite its A-flat major title it begins in A-flat minor: the right hand pours out cascades of broken chords in sixteenths, top to bottom, while the left hand sets the chords and then sings a melody in the middle register. The trio, in C-sharp minor, with its repeats, is darker; then the opening returns and the piece brightens into A-flat major.
Work the cascades by positions: each group of four sixteenths lies under the hand, so you only need to link the positions without noise. Keep them pianissimo, like running water, to let through the left-hand melody, which has to sing above everything. In the trio, the repeated chords of the right hand need a relaxed wrist and a full sound that never turns hard.