Pros: neutrality, quality
Cons: sub bass amplitude
I have searched high and low (literally from $10 to $600) in universal IEMs. I have settled completely on the ER-4S. I found very few IEMs that sounded anywhere near flat. The best three I've found have been the ER-4S (1), the Phonak pfe 112 (2) and the Westone 4R (3).
The PFE 112 and ER-4S are EXTREMELY similar in sound and neutrality except for two slight differences. The 112 has a bit more neutral bass. I don't find the ER-4S lacking really, but the 112 is more right on that flat line for bass sound and measurement wise. However, the 112 has a small spike in the treble somewhere around 10khz I think it was. This gives things a slight raspy quality sometimes and makes the air of the soundstage sound a little softer. It can also accentuate certain hiss that sits in that 10khz band. This is fairly common in 1970's progressive rock. Very slight difference though. As a matter of fact, compared to every other IEM I've tried the differences between the 112 and ER-4S are the smallest of differences. But they are there.
Fit wise the 112 wins as well in comfort and ease of use. However, the final verdict for me is the incredible accuracy of the ER-4S. The 112 is absolutely phenomenal, and thanks to ClieOS for recommending that off the bat. I wouldn't be where I am now if it weren't for going down that trek. Anyhow, the ER-4S is extremely accurate in bass, while only a tad lower in amplitude than the 112, however certain tips make them equal or more varied, depending on your preference. However, the overall spectrum from sub bass to ultra treble is almost perfectly smooth and flat with the ER-4S. Therefore, I hear no flaws with any spikes or dips at all with them. No other set has achieved this. This is incredible.
The Westone 4R are a slightly more distant 3rd. They offer a fairly flat and neutral sound signature, but with some problems in the mid bass and treble. The mid bass has a sort of thick hump that makes them lean towards warm sounding. The problem I have with this is that it really masks that nice ultra detail that you get with the ER-4S. You can EQ the mid bass (100-300hz roughly) down a few decibels and immediately hear the details open up on the 4R. They are also a tad more uneven with a slight boost in certain treble frequencies. But overall, I'd have to call them 3rd place so far with what I've heard. But the mid bass masking really takes them down a lot for me. But since it's such a simple eq adjustment to repair that, their other qualities make them a nice set. They are comfortable, seem fairly durable and have nice accessories.
Anyway, back to the ER-4S. They sound excellent straight out of an iDevice and have ample volume for the most part. However, some of the quietest songs suffer a bit in that area. I grabbed a JDS labs C5 portable amp and it really makes them a step up in quality. It opens up the space between instruments and really brings out the crisp details of the set. It is dead silent at any volume and really allows them sound their best, which is really freaking awesome. And provides more volume than you'll ever need.
There may be a learning curve for fitting them. For anyone interested, I've post some info on this here:
Once you have a good fit/seal, I find these are the closest approximation of my studio monitor speakers I've ever heard without the "feeling" of the bass, which is typical in IEM world. But they just sound so darn good. I've heard the sound of the ER-4P, and I find them very good, but they are essentially the same exact sound with lowered treble. So they sound more warm and therefore the bass "sounds" louder, but it's really just that they lack the treble neutrality of the ER-4S. It's a great option to buy the ER-4P and add the 4S adapter though. Then you get both with the switch of a cord. But personally, I'd never use the 4P with the accuracy of the 4S.
I'm not sure what people are referring too when they talk about small soundstage. I've had a LOT of IEMs in the last six months to try out, and the only time I find the ER-4S to have a small soundstage is when they aren't fit perfectly. When I have the tips deep and sealed well I have full stereo separation and depth. Certain songs with distinct right left separation have instruments that are fully in one ear to the far right or left. The depth and ability to hear "into" the soundstage is equally impressive. I find that some IEMs make a false soundstage by permanently making everything sound a little further away or a little deeper than they should. The ER-4S reproduces everything as I would expect. Songs with depth and distance sound as such. Songs with dryness and forwardness sound equally as such.
I think what they do very well, which might seem like a small soundstage, is that they have a very smooth panning across the stereo field. So the main vocal is dead center, and as instruments are mixed across the sound field, they smoothly span the field as mixed. You will only notice that things are very wide when the song is mixed that way. For instance, in the song "Moonjogger" by Fourplay, the intro has some percussive instruments that are panned hard left and right. They provide a nice wide clear presentation. Also, through the song the cymbals and bongos in the left and right are very well placed. In other songs you might notice the mix is mostly forward with slight panning. So, I think this depends greatly on the music you're listening to, but I have found they are perfectly capable of a small or large soundstage when required by the material.
To sum everything up, I have never heard such a good IEM, and they have fully ended my search. With my C5 amp and ER-4S I'm in auditory heaven. :-) I highly recommend them both and some time to get used to them and break in the tips. They are not for bass heads, and even still I think a lot of people prefer bass elevated above neutral to compensate for the speaker feel. While they may or may not sound neutral in the bass to some, they are the closest I've ever heard to neutral. If there were one single area I think they impress the least, it would be sub-bass in the lowest register. However, they still sound very accurate down very low and most IEMs fail in this area anyway in terms of accuracy. Nonetheless, they have "good" sub bass and with the C5 bass boost the sub bass is phenomenal. Even to the point that I find it to be too much. I think those wanting more sub bass but neutrality would like a 3db boost at most. At least that's the most I would apply. However, I don't use the bass boost and it never bothers me. I get engrossed in the music and find that they sound incredible all around.
The fact that I find them superior to $600 IEMs such as the PFE 232 says something incredible since they are a single driver. The Westone 4R is FOUR drivers, but in my opinion not as accurate or high quality (but still very good). I think that clearly shows that number of drivers isn't everything. If they added one more driver to the ER-4S to gain that perfect sub bass that would be great, but it might be possible without another driver. I'm not sure if it was a choice in tuning or a technical limitation. Nonetheless, the 95+/-% accuracy is probably referring to everything but that 5% or so of sub bass rolloff. So for one driver, that is insane. But I digress...
I'll be posting a video review soon on my youtube channel: www.youtube.com/user/dentreviews