I have both the ER6 and ER4P, and Shure EC2.
First, the quality issue. If you read everything here, you'll see that people have a lot of problems with Shure cords failing. They're bigger: so what? They don't seem to last as long as Ety's "inadequately thin" cords. Just being fat doesn't mean you're strong. End of that issue.
My 6s are 2+ years old, broken in, and sound great (and the cords are fine--my Shure cord broke after about 8 months). My 4s are a month old, and I don't really know where they're headed. The general differences, unamped on my iPod, I would say, is that the 6 is very clear, because of really strong treble response, and the 4 seems to have more body to the sound, at the expense of clarity, relative to the 6. Once you balance this out with an equalizer, the 4 wins on presence and density. The 4 without an amp on my iPod isn't too pleasant to listen to, though--there's something going on in the upper base-lower middle that makes them sound meaty, which fatigues me quickly, so in that mode, I do prefer the 6. With an amp, though, they're much better than the 6, in all respects, with a clearer, more defined bass, and less fatiguing treble.
I'm a transparency freak, so I prefer the Etys. If you're just into comfort, though, you'll prefer Shures. They're much less defined, and don't have the transparency of the Etys, but are more musical. I can't get into them, myself, but I do understand them, if you aren't totally wrapped up in hearing each instrument, instead of the overall, and I do understand why so many people like the less analytical, more friendly, Shures.
So, my summary: unamped, Shure or Ety6, depending on how you feel about detail vs musicality. Amped, Ety4.