Chromaticism can be explained in this piece by the use of non-harmonic tones (passing tones, appoggiaturas, etc.). Let's examine each voice to understand how Chopin goes from the V (measure 2) chord to the V / IV( measure 4).

Lower voice - The F# in the bass moves to the E in measure 4 using a chromatic passing tone (F natural). Note that the chord in measure 3 could also be explained as a French augmented sixth (F - A - B - D#) of A minor or a V/IV with a diminished 5th. The B is played by the right hand.

Upper voice - The D# (written as Eb in some editions) moves to D natural (anticipation) in the second beat of measure 3.

Middle voice - The A moves to G# (anticipation) in the second half of the second beat of measure 3.

As you can see, Chopin does not moves all the voices at the same time. He first moves the lower voice (measure 3, beat 1), then the upper voice (m 3, b 2) and finally the middle voice emphasizing chromaticism and creating very interesting chords.