The ER-4 gets a bad rap that it really doesn't deserve. I've clocked in about 75 - 100 flights in the past 20 years, in addition to endless noisy train rides, airports and train stations I've had to either work or (ugh) spend the night in. I'm still using the same ER-4 I bought 7 years ago, and they're still as perfect as ever: precise, utterly accurate, totally balanced sound that is forever non-fatiguing, even after literally 1000s of hours of use.
In addition, when I had an out-of-warranty disaster last year, (I accidentally cut my cable - don't ask...), Etymotic rewired the whole thing to to bottom, put in new filters, added a totally upgraded cable, AND replaced BOTH transducers... for $50. Even out of warranty. And I had them back in my ears inside of a single week, to boot.
The ER-4 having no bass is really a misnomer - it's not totally true. When I'm spending 15 straight hours in a tiny chair on a flying bus, I bring along my BSG CMoy amp, and using my S cable, suddenly the ER-4 is delivering pretty slamming bass. The BSG, as well as other amps I've used them with, also vastly increase the soundstage to the point where they even rivaled my K701 - and impressive claim, but true nonetheless. So they scale extremely well, and the better your amp, the better their sound - including bass.
Is it an M50? Well, no. Not even close. But it has better bass, when amped, than my K701 and AD900, for example. And the ER-4 delivers, year after year. And with the ER-4s in my ears, I can watch a movie, listen to a speech, or relax to a quiet symphony and I even forget I'm on a plane at all because I can no longer hear the incessantly annoying engine whine. Or the crappy but LOUD stereo system at my local gym.
What other IEM can do that? For <$200 these days to boot.
And with my BSG (portable) or better amp and some hot-out-of-Ibiza Vocal Trance, I'm booming away in an island club, instead of packed into a seat the size of a bicycle saddle, next to a snoring old man and a screaming baby.
Like any headphone, they just have to be properly amped to achieve their fullest potential. But they work just as well, and keep you in utter silence, straight out of an iPod - wherever you happen to be, that you need quiet. All this in an IEM that has one of the best frequency graphs, and one of the most supportive and respected companies, of any out there.