Pros: flat with a roll off in the treble
Cons: needs more treble extension
Imagine the flattest IEM that ever existed, take it and add a roll off to make it less clinical and more musical. You have the ER-4P.
The ER-4P is never harsh sounding, or even clinical... I would classify it as slightly dark sounding despite what many think. The bass hits hard with no noticeable roll off. One of its weaknesses is probably the lack of soundstage, however the pinpoint imaging is very good.
Its major weakness is the treble roll off, but I'm not sure whether I'd call that a weakness as it was somewhat intentional I believe. When compared to the ER-4S, the ER-4P has more body in the sound and a recession in the upper treble... think cymbals and stuff. The ER-4S is also more detailed and the bass is perceived to be less.