Well, hongman, I have the ER6 and the ER4s. I had the 4Ps until my dog did them in. The sound of all three is typical Ety sound. A person with an untrained ear might be hard put to differentiate between the 3. To my ears, the 4 series has a more refined, airier sound quality with very deep bass. Of the two, most people would agree that the 4S provides a more accurate sound. The 4Ps, being much more efficient, are very revealing of sources (hiss, etc.), with just a tad grainier sound quality. If you go for the 4S, I would get the 4P and the cable to transform them into $s when the need or desire arises.
As for the 6s, I've been listening to them as my main 'phone since I got them Wednesday. They are VERY close to the 4 sound. Not as refined in the top end, and a bit more forward presentation. I think fjhuerta characterized them as "Etys for Grado lovers." That pretty much nails it for me. The bass seems more prominent than the 4S, and is about the same as the 4P, to my ears, but it doesn't seem to go as deep. Still sounds great though.
My only complaint with the 6s is that it is not as easy for me to get a good seal. The shorter silicon tip makes it difficult to grab hold of the plastic transducer housing when inserting them. It can take me awhile to get a seal, whereas the 4s, for me anyway, are plug iand go. Hope this helps.