Etymotic, a Lucid Audio Company - What goes into an ER4 earphone? Take a look for yourself! - Balanced Armature Driver (speaker) - Braided Cable for reduced friction - Green Filter https://www.etymotic.com/consumer/earphones/ er4-micropro-earphones-373 ...
Kings of clarity: Etymotic ER4 in-ear headphones - CNET
If you still love Etymotic ER4, this is the thread for you... | Headphone Reviews and Discussion - Head-Fi.org
Etymotic Research ER4SR Studio Reference Earphones | Guitar Center