Leaky74 posted a review of the tweaked ER4 called the ER4-XR over on Head-Fi. Generally speaking, if you want extreme detail and a ruler flat SQ in your IEM you buy an ER4. The only complaint over the years is with the bass response. This new version has tweaked the bass up notch to address this complaint.
The added bonus of having these crammed down your ear canal is you receive a level of isolation and seperation which is unparalled in the industry.
Pros: Stunning levels of real detail, clarity & separation.
Cons: None to speak of; maybe a little clinical for some.