http://www-personal.umich.edu/~siglind/wtc-i-24.html

このドイツ人の学者さんの本の内容だと、4小節目まではB minorって解釈してるね

The first harmonic progression concludes in bar 4.
The lower voice, after the prominent octave jump on the dominant note F#, ascends in a segment of the melodic minor scale until the keynote B on bar 4d.
The upper voice, however, diverts its expected course at this very moment with a jump to the F# before reaching the keynote on the second beat,
which is also where the middle voice resolves its suspension.
Thematically, this cadence marks (belatedly) the end of the initial statement of the extended motive in the upper voice.
In its actual position of overlap, it can certainly not be regarded as a structural caesura.

In the first section, only the initial phrase contains such subphrasing (on the downbeat of bar 4),
yet conversely to all other subphrases mentioned, this one is not harmonically active but simply confirms the tonic.

いまいち英語できないからあやふやだけど、多分ここらへんが言及してるところかな。
突然転調の考え方はちょっと、柔軟性にかけるというか教科書にとらわれすぎだと思う。
誰かこのページ全部翻訳してほしいｗ