They're different from the ER4S, not only in sound but mostly in a less penetrating fit, which will make the price worth it for some.
They don't sound quite as clear (but clearer than anything else I've used) or quite as treble-tilted, which I think is a product of how the mid-range boost was achieved using the titanium acoustic horn, and how it might increase distortion in certain frequencies. That approach might also have a larger effect on fit, I'm not sure. Different tips could change these observations.
I'd love to get these graphed.