Ah, posts like these bring forth Head-Fi's most respected and knowledgeable members. As a reference IEM (according to Etymotics goals), I think the ER-4 is their greatest accomplishment and considered perfection in their books. I love my ER-4S and absolutely nothing replaces it. To me, the ER-4 is what music "should" sound like. But everyone gets bored of that very easily so people from Shure, Westone and many more decided to sell IEMs with multiple drivers to make the complete opposite of what music "should" sound like and it was an absolute hit! Honestly, I really like some of the products they created (Shure SE535, Westone UM2, Audeo PFE232 just to name a few) because they filled in the gap of listening pleasure. They achieved what Etymotic couldn't (wouldn't?) achieve and that was a specific sound signature per IEM in their lineup. Think, what Shure, Westone, Earsonics IEMs all sound the same? They might sound similar, but never the same because WHO WANTS TO BUY THE SAME SOUND OVER AND OVER AGAIN? Marketing people, that's why. Compare the UM2 to the UM3X. Very different IEMs targeted at very different potential buyers. I might be very Etymotic biased here, but Etymotic to me is like the ultimate "reference" point in the IEM market. It's sort of a centerpoint. Of course the ER-4S isn't perfect in anyway (cough cough fit and comfort), but to me it's the closest thing to the ultimate reference monitor. The only thing that comes close is the RE272 and possibly a few CIEMs, but even those sound a bit emphasized here and there. ER-4 is no excess, no limiting.
Just an opinion from a very bored person on a very hot night in very normal vancouver.