Plug the ER4 into your mobile phone and if you need to push the volume to almost max or max volume to get a decent level then chances are you have an ER4S. ER4P is more efficient so by pushing your mobile phone volume to almost max or max volume, you'll probably blow your ear drums. You only need half volume or slightly above half to achieve a decent listening level on your ER4P. Hope this helps.
Edited by Francisk - 7/12/14 at 3:46am