OK, in a nutshell:
The ER-4P have a very sharp and well-defined sound, so sharp and defined in fact that it may seem a bit unnatural. They have a very small, compressed soundstage, but within that soundstage, they image and resolve exceptionally well. Instrument separation is excellent, and at times, it's almost too much - as if you're listening to multiple parallel recordings rather than one thing.
They're not smooth. There is a lower treble/upper midrange glare and a very sharp upper treble, made even sharper with a bad seal. They will sound smooth with very smooth, laid-back recordings, but play anything with sharpness or sibilance (i.e. electronically amplified/distorted guitar), and you'll tire your ears out pretty fast. With acoustic music, that's not a problem, and the treble has the added benefit of giving extra life and sparkle to your recordings. With most ambient music, the treble really brings out hidden detail, but sharp high-pitched sounds will sound grating.
The bass is definitely there, but it's not there in excess quantity. Depending on your definition of excess, you might find it lacking. If you want to hear all the detail and nuance in your bass, this will do nicely. If you want a solid, palpable backbone to groove along to, there won't be enough of it. But, if you use EQ correctly (drop mids/highs and raise volume), you can boost the bass quite a bit. The Ety's are capable of fairly big bass - they're just not equalized that way.
If I were to make an analogy, I'd say that the ER-4P is like a scalpel for dissecting the music. They break everything down into it's constituent components, and serve it to you so that each and every individual component is readily apparent. I guess you could call them "analytical"
I do not use this, however, in a negative sense. Some music, like electronica, sounds really amazing on them. They really do not sound thin - they're just unnaturally accurate.
The only really smooth, full, coherent portable - outside of custom-molded IEM's - that retains detail and clarity is the Stax SR-001. It also has a rolled-off treble, but it doesn't lose much detail because of it.