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.
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.