They base the ER-4 on extended research, based on the average human-related transfer, thus their ideal curve. This approach has actually been done by Beyerdynamic, Stax, Sennhessier in their full-size headphones. . By getting as close to the curve as they can they did create something special, 95% of the IEMs I've heard are loosely tuned, at least the manufacturer has backed research involved and the performance shows in certain areas.
Is there room for improvement? I believe so, specifically at the extreme frequency ranges. A good way for them to keep their curve while getting a more visceral bass presentation, will be to introduce a dedicated woofer, a real "woofer", one that will increase regions only below 100hz. It will compensate for that lack of full-body vibrations in IEMs, while keeping their transparent bass presentation in the midbass. Then a dedicated tweeter to maximize treble bandwidth up to 20k, though the latter here will be be a small increment. Perhaps find a way to maximize soundstage width while retaining their class-leading isolation. I think the latter point here is mostly overlooked by them as the benefits of isolation when commuting far outweight the cons of giving that up for small stage width increases. I though the Phiaton PS200 did a fine job to compromise between the two though.
MC, HF and ER6 series are with a limited budget in mind and portability. I find the HF series to be easily one of it's best at it's price-point. ER4s aren't overpriced at all considering the actually sonic QC going on and most multi-BA IME compromise more in performance.
IMO IEMs by design are very limited, thus cannot reach as close to "perfection" as a transparent loudspeaker set-up will.
ER4s have better driver control-quality compared to the HF5s. The drivers are more closely matched and they have less distortion. Much much better build quality as well. I do think the 4P have slightly sonic characteristics as well as in more subbass and treble extension. Rin may get the chance to look into this closely. Same filters, same drivers but still have notable differences, though not extreme. 4P are targeted more towards the "purists".
Edited by Inks - 8/8/12 at 11:31am