The ER4S came in today. I told James I wasn't going to be able to get to it, but who am I to resist the veteran of the iem world.
I feel like the ER4S can be summed up with one sentence..."Dat upper midrange bump."
It confers the ER4S' greatest strengths, as well as its biggest downfalls. The ER4S is the first iem I've heard, other than the T-Peos H-100, that truly gave the the impression of being "cold". It's not even a bass issue either, as I'm getting clean and deep bass. The midrange hump reeeaallyyy emphasizes every last micro detail and harmonic, but there's little body to back it up, making notes that fall in the area of emphasis seem like they were machined from a sharp blade. This heavily applies to guitar strums. At the moment, all I can say is that I think the HF5 might just be the best all-around iem in the Ety lineup. It's not a fit issue either...I learned my lesson with the HF5.
Of course, these are very initial impressions. I'll listen more after my exams.