I'm pretty much with Teerawit here. I think the ER-4S does sound pretty good out of the iPod, though the volume is definitely limited. Both the ER-4P and the ER-4S improve a lot with an amp, so saving a few bucks to put toward that amp is not a bad idea.
I found I just couldn't listen happily to the ER-4P. The tonal balance is off enough to really bother me, so I wound up using the 4P->4S cable full time, even unamped.
Another alternative, well worth considering, is to go with the ER-4P and plan on buying one of Xin's amps that has an internal 4P->4S converter. The small size is the next best thing to going ampless, and the amp delivers a lot of sonic improvement.
Finally, something you should know is that the 4P and 4S have different cabling (or did the last time I checked with Etymotic a few months ago). The 4S has a somewhat rubbery cable leading to the Y-connection, and rubbery cables leading away from it. The 4P has a plastic-coated cable leading to the Y-connection, and much, much lighter braided cables leading away from it to the earpieces. I find the lightweight braided cable less of a hassle to deal with than the rubbery cable, so on the whole I'm happier with the 4P plus adaptor than I would be with the stock 4S. But your mileage may vary; some people feel the rubbery cable is less microphonic.