Couple of points:
You may have what Audiologists describe as a ‘dead-spot’ in your right ear - no matter how much your audiologist turns up the sound you can’t decode it. Think of a piano actually missing half a dozen keys in one area just above or around top ‘C’.
No matter how hard the pianist (hearing aid) tries to hit the notes, you still don’t get the right sound. However in trying to hit the notes harder (a la Little Richard with his foot), he’s also hitting more and more of the surrounding notes which is causing a drop in the resolution of the signal. Furthermore playing a piano (your cochlear) with one’s feet causes the whole system to resonate and affect notes further down the scale (downward spread of masking) thereby degrading the signal clarity even further for the notes you were ‘getting’ before.
Remedies - Accept you’re never going to hear properly at 4Khz in your right ear. Turn the aid off in this area. Return the rest of the hearing aid to normal ‘comfortable’ gain settings and possibly edge-up the gain at frequencies a fraction lower and higher than the ‘dead-spot’ to preserve overall signal quality.
Also NB that the Rexton uses the Starkey feedback canceller that according to Dr Gus is the best thing since sliced bread: however if it jumps all over the HF gain where you need it, it’s not the best aid for you. Try an alternative if you feel that the adjustments above aren’t effective - especially since other aids may offer better channel resolution either side of your dead spot with fewer issues.