In J.S. Bach’s Two-part Inventions both voices overlap and imitate each other creating counterpoint.
The SUBJECT of no. 4 contains a d-minor Harmonic form scale whose 6th note, B flat does NOT continue in an upward motion to the leading tone, C# or 7th note, but instead, the C# is displaced down to the lower one. (B flat goes down to C#) This is unexpected in the course of scale progressions, so it has an emotional impact bound up in the intrinsic nature of the interval and its fall. (watch phrasing, and roll wrist forward for the ascending scale)
In truth, the descent sounds generically like a Major 6th, but its spelling conforms to a 7-letter spread, making it a 7th.
The second part of the opening subject, is the broken-chord, detached 8th-notes. They should not be too short. (I think press/lift)
Once the content of the Subject is understood, then any elaborations should be noted as occurs starting in measure 5 and on, as well as sequential measures, where a melodic or bass segment may be repeated a step below or above–or for that matter any uniform distance as long as there’s a symmetrical relationship between measures or phrases. (melodic and/or harmonic component–rhythmic as well)
The trills spelled out in the Palmer edition, are not played rapidly. They’re designated as treble 32nds against 16ths in the bass. When the trill is reversed, the Left Hand plays 32nds against 16ths in the Right Hand. (These would be called “measured” trills)
A very poignant juncture is at m. 48, with its DECEPTIVE cadence. An awareness of this surprising emotional shift is needed, so be prepared for an unexpected delay by way of a Bb VI chord.
Above all, carefully shape phrases and be aware of the counterpoint at all times.