To keep it short, the 4ai does have a slight bass boost to give it that sense of naturalness, while enjoyable it is not neutral in the sense I find it 100% accurate. There is also a narrow midrange suckout that affects the coherence of strings like the guitar in Hotel California from Hell Freezes over for example. That's all I can say for now as I'm on the loaner program for the 3ai and 4ai and need to decide how i will be passing along my impressions.
The 4ai sounds quite different from the ER4S or UERM.
So, if there is a bass boost for naturalness, wouldn't that be natural and truly what flat should be? Not to say you are wrong, but just asking a question.
In my experience, that would be the NT-6...not warm or enhanced, but oh so capable in the bass region if necessary. However, the treble is north of neutral/flat to my ears.