It might seem like you'd weaken the effect of the entrance in m. 5, but that's somewhat of a "presentist" viewpoint. Bach didn't seem to think it would weaken it (because that's how he wrote it), and the countless performers who have recorded it don't seem to think it did, either. One reason might be because of the rhythmic activity: since m. 5 is the first time eighth notes appear, that's enough to highlight the re-entrance of the subject.

And unfortunately, the countersubject in mm. 5–8 doesn't match the figures given in mm. 1–4, so overlaying that countersubject in the first four bars wouldn't match the chords. In some cases, it would create some terrible dissonances!

Fortunately, the Peters edition edited by Ferdinand David has a realization; it's available at IMSLP.

It might seem like you'd weaken the effect of the entrance in m. 5, but that's somewhat of a "presentist" viewpoint. Bach didn't seem to think it would weaken it (because that's how he wrote it), and the countless performers who have recorded it don't seem to think it did, either. One reason might be because of the rhythmic activity: since m. 5 is the first time eighth notes appear, that's enough to highlight the re-entrance of the subject.

Although transposing the countersubject in mm. 5–8 would fit with the progression given in mm. 1–4, the fact is that that's not what Bach notated. So no one is stopping you from doing that, but it would go against performance practice. The better option would be to realize the figured bass in the first four measures. (It's also not as common to start with the subject and countersubject at the time same, although it does happen occasionally.)

Fortunately, the Peters edition edited by Ferdinand David has a realization; it's available at IMSLP.

It might seem like you'd weaken the effect of the entrance in m. 5, but that's somewhat of a "presentist" viewpoint. Bach didn't seem to think it would weaken it (because that's how he wrote it), and the countless performers who have recorded it don't seem to think it did, either. One reason might be because of the rhythmic activity: since m. 5 is the first time eighth notes appear, that's enough to highlight the re-entrance of the subject.

And unfortunately, the countersubject in mm. 5–8 doesn't match the figures given in mm. 1–4, so overlaying that countersubject in the first four bars wouldn't match the chords. In some cases, it would create some terrible dissonances!

I don't know of any existing realizations, unfortunately, but figured bass isn't too terribly hard. At its most basic, it literally tells us what intervals to play above the bass. A seasoned performer could read these four measures at sight, but you can certainly work it out on your own on staff paper if that helps.

It might seem like you'd weaken the effect of the entrance in m. 5, but that's somewhat of a "presentist" viewpoint. Bach didn't seem to think it would weaken it (because that's how he wrote it), and the countless performers who have recorded it don't seem to think it did, either. One reason might be because of the rhythmic activity: since m. 5 is the first time eighth notes appear, that's enough to highlight the re-entrance of the subject.

And unfortunately, the countersubject in mm. 5–8 doesn't match the figures given in mm. 1–4, so overlaying that countersubject in the first four bars wouldn't match the chords. In some cases, it would create some terrible dissonances!

Fortunately, the Peters edition edited by Ferdinand David has a realization; it's available at IMSLP.