Ok guys... Here are some initial impressions. I haven't got excited over IEMs/CIEMs for a long time. Last time I got excited was because of 1+2. But now I can say I am soooo glad I can hear these bad boys.
Source: Anedio D2.
Tips: I tried several, I find the stock one to be decent enough, will try more later.
I will skip all the fancy words and get straight to the point. But THIS IS JUST MY OPINION.
Stage 4:
I talk about it first because I found 4 demo is the the most colored IEM out of the three. Overall it's more like a fun sounding IEM to my ears. I agree with most people here, Stage 4 is slightly V shaped. But overall it's detailed and smooth in terms of sound signature. It has a slight lifted mid-treble (11- 14k Hz) which doesn't bother me much as it makes Stage 4 sounds detailed and sparkle. But the hump at upper bass - lower mid is where the majority of the unnaturalness comes in. It not only enhanced the drums, but also makes every vocal I listen to cover with a extra layer of vocal bass. Bass extension is decent. Although overall the bass is definitely enhanced, it didn't impressive me much. Another reason why Stage 4 is V shaped is because of it's recessed upper mids. The vocal didn't have much presence/clearness compared to the other two. IMO, 4 is more suitable for stage use rather than listening to music.
Stage 3:
Stage 3 demo is a very neutral IEM to my ears. Definitely the clearest in the group as Joe said, but also the thinnest too. It has a very spacious and clean sound signature. Mid and treble are much more linear compared to Stage 4. Treble extension is on par with 4 too. It integrated beautifully with the mid resulting in accurate/neutral imaging, soundstage and depth. There is just really nothing I dislike about the mid and treble. However the bass is on the lighter side of neutral to my ears which might be because of the demos? I didn't get that "hollow bass feeling“ in JH13 due to its imbalanced between sub-bass and mid-upper bass, it's just not dominant overall. Extension is OK. But in terms of capacity and impact, it's not on par with other neutral CIEMs like NT6 or SE5. If Stage 3 has NT6's bass and more mid presentation, I would call it the most accurate sounding CIEM (above NT6) I have ever heard.
Update: the bass improved a lot with smaller tips that go deeper~ But still the lightest within the three.
Stage 2:
From Stage 3, I realized that number of driver doesn't mean anything in CIEM world. Stage 2 really shocked me how much potential two drivers can deliver. Maybe it's source I use, I don't know. It sounds absolutely incredible as a 2-driver CIEM! It's as balanced as the 3 to my ears if not more. 3 is more like mixing/reference type of IEM, while 2 is more like a music listening IEM. The bass impact is decent. Mid is closer to 3, treble is extended and detailed sharing both 3 and 4's characters. I don't want to hype joe's comment, but this proves once again how accurate he can hear things. Stage 2 just place you right in the music without knowing you are listening to a CIEMs. It just sounds so right. I don't know if I can pick any flaw in terms of frequency balance. It just fits my taste so well. If I have to pick anything, the presentation a little bit more forward than "neutral". Technically, of course it's not on the level of SE5 or NT6 in terms of ultimate extension, detail, bass control and resolution. However it would be such a good choice for daily music listening.
Overall I would rate Stage 2 > Stage 3>>>>Stage4. THAT'S JUST MY OPINION.
Take my word with a pinch of salt because sound can vary a lot upon different tips.
but they don't tell me what's the signature of it.
I have to say I really enjoyed S2 and S3 a lot~ 
