I've delayed writing a review due to a cold but to me its a wider sound, more airy and clear.
N3i
Darker more compressed sound
Vocals aren't as crisp and are more sibilant
Bass is more present and more boomy
Midrange is slightly more recessed
TD4
Vast difference in bass, not as powerfull but has a punch and extends deep. It doesn't intrude into the other frequencies and is present where needed. I guess its how it should be, very similar to memory of the Coppers bass. On other Xears models, the strain reliefs acted as vents but with a thicker cable there is less ventilation so less bass.
More forward midrange, more detail better clarity
Still a bit sibilant but hardly noticeable to me.
The cable and build are fantastic, have a really nice weight to them and dont tangle easily but no cable cinch. There is also less driver flex and seem like a well put together IEM.
Overall i prefer the openness of the TD4 as a basshead its a nice change. Although i prefer the bass to knock me of my seat there is enough to keep me happy. Its a matter of taste here, those who want bass monsters, look elsewhere but those who want a punchy and clear pair with fantastic build, these are ideal.
Hopefully I can add more comments about the copper this week, my friend who owns them is coming down to see me.