I don't own Ety's (yet) but I did listen comparatively to all 3 models at the Headroom tour at length, and thought the differences between the er-6 and er-4 to be quite immense. Now, admittedly, perhaps I didn't fit them properly, perhaps this was just the effect of newness, and my ears are not as headphonically educated as many others here. YMMV, etc.
Nevertheless, subjectively, I thought the ER-4 models sounded much much better in every dimension. I was worried from comments on boards that I mind find them excessively clinical, and worse yet "weak on bass" -- but instead I found them balanced and profoundly musical and interesting (I can't say I found them comfortable, mind you, but many here say you get used to the feeling of having your ears completely stuffed with foreign objects.) The low end on the ER-4x struck me as tight and well-defined and extremely appealing subjectively. To your actual question, of course, I found the ER-4 felt "fuller" in my ears, comfort wise, and thought the ER-6 were actually more comfortable, alas.
The ER-6 I found trebly, I don't dare say tinny, but shallower and harsher in the mid-to-high in a way that seemed much more mundane at least in immediate comparison, the way mid range Sony's sound compared to higher end Sennheiser. Maybe I just had a better seal on the 4 series, but both the 4P through my own portable source and 4S through Dan's PortaCorda took me a lot further.
I went expecting to leave and order the Etys--and I do hope to own a pair eventually, as they are great phones. But then I had a very unexpected religious conversion to the HD-600s and bought a pair the next day. And it is scary how much I'm loving these phones...
Fwiw, AM