HI: The er-4p are easy to drive out of a portable source and have nice bass. The er-4s sounds better with and amp out of a portable but even better with and amp and better source. So by getting the er-4p and adapter you will have the choice of both the etys. The er-4p on the go with portable and the er-4s when at home with amp and better source.
Okay, I'm not really sure where to post this since it's related to everything, but I already started this thread so here goes...
The two portable audio systems I'm deciding between:
#1) Sony D-EJ955-->Porta Corda-->Ultrasone HFI-650
#2) Pansonic SL-CT570--> Super Mini-->4-P to 4-S Cable-->Ety ER4-P
The SL-CT570 for #2 because it's cheaper, and the mini because that's cheaper too and super small. I was almost decided on #1, but the Etys and super mini would just be so easy to carry around and a lot less...conspicuous I guess? #2 would be definitely more expensive...maybe if I didn't get the cable I could still get the D-EJ955? I'm still not sure if I should just get the ER4-S or not.
If you're already absolutely set on getting an amp, particularly one that's portable, then by all means save on the cost of the 4p-to-4s adapter and just go straight for the 4s. Make sure that the amp is portable, though, and that you'll have no gripes with carrying it around wherever you go. With the 4s, you need an amp, whilst with the 4p, you can just casually plug it straight into the portable's jack and you'd only be carrying the pcdp, nothing else.