All the Neapolitan comments seem off the mark, because it isn't really functioning like a subdominant `N6` that would normally move to a dominant chord of some kind. In other words, it isn't followed by a `G#7`, so it isn't a convincing subdominant to dominant in a minor key movement.

But, there is a very obvious melodic sequence happening, and the pattern of that sequence is a decoration of a third, an implied root position triad, that leaps up a _perfect fourth_. Not just any old fourth, but specifically a perfect fourth.

Two important points should now be mentioned: 

  - melodic/harmonic sequences are frequently used to modulate to new keys
  - the perfect fourth above a tonic is an important tonal degree and chromatic alterations that form that tone are important in modulation

The second point I think is less in effect for this passage, but at the very least when we get to the third measure, the `D4♮` really stands out as a perfect fourth above the outlined `A C#`, and it has a strong `A` major feel. The `D4♮` rather than sounding like solfege `RE` lowered in `C#` minor, sounds like `FA` in `A` major, even if that event is just a temporary shift. The sequence continues, so at this point we wouldn't want to settle on `A` major as a new tonality.

The first point is probably the more important one. When you see sequential passages with accidentals, it should signal you to look for a modulation or temporary shift of tonality. When such changes aren't modulations, then it's most likely about _altered_ chords, and altered chord usually stand out for their particular "sounds" and often has conventional resolutions.

Let's continue with the premise it is modulating.

The passage stops on `C#4`.

The natural on `D4` has the temporary effect of a key signature of three sharps, which would be `F#` minor.

`C#4` is the dominant of `F#` minor.

The decorated third of `G#3 B3` in the fourth measure outlines a `iio` chord in `F#` minor.

`iio V` makes a nice half cadence in `F#` minor. From a common practice tonal perspective that conventional harmonic progression makes much more sense than a supposed `N6` chord moving to an implied minor dominant to tonic move, which would not be conventional common practice harmony.