A900 is a lot more exciting than the ER-4S, even when it's unamped. It's easy to drive, and you can get a lot of very punchy sound out of it. It's a very resolved headphone, but it's not quite as clear as the ER-4S. If you're going for unamped, I would definitely recommend A900 over ER-4S, but A900 is far from what I would consider portable.
Even with an amp, I would prefer A900 over ER-4S, but that has more to with the sound signature. It sounds like a little brother to the CD3000, doesn't extend quite as far either in highs nor lows, a little muddier in resolution, but sounds very alike and very close quality wise.
I would consider ER-4S much better than the Shure E2. E2 has a good midrange, but its bass, although forward and apparent, is pretty muddy. In comparison, it doesn't have much of any accuracy in the bass region... but still much better than most earbuds out there. Either the A900 or the ER-4S would beat up the E2, in just about any setting, amped or not. In fact, I don't think amp helps the E2 very much at all... It tightens up the bass a bit, but the earphone itself just lacks high-end extension.
Edit:
Forgot one more thing, I'd still consider the E2 more "warm" than both the A900 and the ER-4S. Personally I feel you'll be happier with the A900... unless you really need portability.