You are making a big mistake if you buy the ER4Ps over the Atrio M5s. I owned the former, sold them for the latter, and never looked back. The Atrios (in my opinion and I am not alone) are by far the best IEMs for unamped use under $200, not even close. I AB'd them with the ER4Ps over and over, and only one result, the Atrios win.
You can get the Atrios on ebay (via seller roaddogonline) for $139 shipped, using the "make an offer" option. Offer $130. I just got back from a 50-minute walk with the Atrios and my unamped CVM, and it was pure sonic bliss. And from what I hear on my daughter's umodded iPod, you will need all clean, smooth bass you can get. And that's the Atrio's wheelhouse ... smooth, clean, impactful bass, decent mids and highs, and most of all, no listening fatigue or sibilance whatsoever. Try that with the ER4Ps. Not happening.
Just read over the Atrio M5 threads. I hope you haven't pulled the trigger yet. If so, send them back and get the Atrios.
And if you get the Atrios, get some Shure olive tips (mediums seem to work best for most people), invert them, and you are set.