Maybe the ER4 driver is a bit too tiny and and the sound waves are too shrunk down to sound properly. The driver size certainly seems to impact its dynamic range.
It's a deep fitting IEM i have no idea why people say the ER4 driver too small to work, When it uses 5 mm BA drivers. The ER4 with EQ, Still outperforms IEM's that use 7mm and above dynamic drivers. Even with a 6db bass boost my ER3SE bass still sounds way more clean/tight than the ER2SE. The old ER4S/B had much better treble quality and extension than DD IEM's, A lot of 7 ~ 13mm has roll of after 10k.
I've no idea why driver size is assumed to play a role when even the HD600 & others kick butt with 40mm Dynamic drivers. By that logic the ER2XR should be struggling since its driver is 5mm?.