Pros: Great fit, nice cable, pleasant sonic signature (non-fatiguing)
Cons: upper/mid bass still bloated, sub-bass not enough, not enough air
I have put the W4 through its paces, and after the first round of extensive testing, here are my impressions.
My main reason for getting the W4 is to hopefully replace the SE535 and W3, since I find them too colored or fatiguing/bright without EQ. Ideally, I wanted a pair of IEM's that I can listen to without any EQ. In a way, the W4 gives me that, but not in the way I had hoped.
First of all, the W4 is definitely much better than the W3, since it's obvious that Westone listened to all the complaints about W3 and tuned the W4 according to those complaints, while they also tried to keep some of what people loved about the W3. So the result is like a compromise between the W3 and what I consider a neutral and accurate sonic signature--in other words, the W4 is still colored, but just not as severe as the W3. To give you an idea of the kind of EQ curve it takes to get the W4 close to the LCD-2's sonic signature, here's a screenshot:
I can probably refine this EQ curve even more and get it even closer, but for now, this already gets pretty close (though missing a bit of clarity/resolution/punchiness overall across all frequency ranges, and I think it may be because of the differences in the inherent physical characteristics of the two driver technologies). For those of you who own both the LCD-2 and W4, give this EQ curve a try, if you want your W4 to sound more like the LCD-2 (which IMO is definitely a superior headphone in all ways possible).
Compared to the W3, the W4's bass has a similar shape, but only half as prominent. That doesn't mean its bass is neutral/flat/accurate though, since W4's bass is still significantly emphasized, but just not to the degree of W3 (about halfway between W3 and neutral). If you listen to any music where the accuracy of the bass response is critical, it becomes immediately obvious the bass is still too prominent. Sparse jazz arrangements that contains a double-bass makes this very obvious. The W4 renders double-bass with too much bloom/mud compared to a much more accurate headphone like the LCD-2. I also gave the W4 a boost in the lowest sub-bass, because I found it lacking some body down there (though the W3 didn't seem to need that boost). The cut in the 125Hz range is exactly half the amount of cut I use for the W3, which tells me that the W4's bass emphasis is exactly half of the W3, but still 4 dB too prominent. Compared to the SE535, the W4's bass is definitely not as accurate/neutral. SE535's bass is actually one of its strongest features, since it's very close to being neutral.
The mids pretty good for the most part, although that pesky 7KHz ear canal resonance is there, just like with most IEM's. Part of me wishes that IEM engineers simply just design around that ear canal resonance, but I understand why they don't--it's because we all have different shaped ear canals. If you use my EQ curve and turn it on and off, it's very obvious how of a difference a steep narrow cut in the 7KHz region makes. Without that cut, all cymbals, hi-hats, shakers, tamborines...etc sound really congested and distorted. This may not be obvious if you have never listened to the W4 with my EQ curve, but once you do, you can't not notice it--it's as plain as day. Compared to the W3, W4's mids are not nearly as fatiguing/bright in the 7KHz range. Compared to the SE535, the W4 is less colored, since I have always felt that the SE535's mids are too emphasized, to the point of being fatiguing and overly bright.
The treble needs about the same amount of boost as W3 in the 13KHz range for a bit more air, so they are actually kind of similar. Compared to the SE535, the W4's treble is a bit more clear/prominent, since the SE535 would require a bigger boost in the 13KHz range to sound more neutral.
With all that said, I think the W4 is the first IEM I have owned that I could listen to without using any EQ and not have a frown on my face. While it is indeed colored without EQ, it is colored in a way that is fairly pleasing and acceptable for a lot of music. But most importantly, it does not break my first rule of audio devices, which is to first and foremost, do no harm. The W4 is not fatiguing/overly bright, and although on some bright material the level of sibilance is noticeably higher than better headphones like the LCD-2, it's not to the point of being painful like many other headphones. The bass emphasis while noticeable, actually is quite acceptable for most musical material, and it's only when you get into really refined music like jazz and classical do you notice it and want to turn the EQ on to smooth out the bass response so it's not overpowering what shouldn't be overshadowed.
I think it's very likely I'll sell my SE535 and W3, since the W4 is the only one among the three I could actually listen to without any EQ and not feel annoyed by the excessive brightness. While the W4 isn't perfect, and in general I'm disappointed that it still requires significant EQ'ing to get close to sounding neutral, but at least it is colored in a way that is acceptable to me. I think my IEM journey ends here, until one day I hear something much closer to my idea of neutral, but I'm not going out of my way to audition any more IEM's, since I really don't use IEM's that much anyway.
EDIT: After investigating further into the different tips and their effects on the W4, I discovered that the tips effect the sonic signature of the W4 more than any other IEM I have used in the past. The peak resonance in the upper-mids and the treble are the most significant changes. With the Triple-flange that has only the stalk cut off, the upper-mids are much more tame and does not require that cut at 7KHz, while the treble is also softer as well (this is comparing to triple-flange with the smallest flange and the stalk cut off). The EQ curve I use now for the triple-flange (stalk cut off) is this:
The largest comply grey sticky foam tips are also good, but I hate any kind of foam tips where you have to wait for it to expand for a seal. It's just an extra step that's kind of annoying. If I'm out and about, and every few minutes someone wants to say something to me, I'd have to pull out one ear, talk to the person, and then put it back in and then wait for the damn foam to expand. After doing that a few times, it's just too much. The silicon tips are far more convenient.
The hard semi-transparent tips I just can't use at all. Never was able to get them to sit right in the ear or seal properly, even as far back as my Shure E4C days. I don't know why companies even include those because I really wonder if anyone uses those at all.