The ER-4S was finicky. Most complaints revolved around it sounding too thin, having too much emphasis around the pinna gain region 2-3kHz, and having a small spike in the treble. The secret to using the ER-4S was super deep insertion, usually with the treble flange penetrator tips. Naturally, most people did not like the feeling of having their ear holes penetrated so deeply.
(mod edit: removing affiliate links)