As the story about IEM began, it was back in around the mid 80’s that a few rock stars began to invest into the development of IEMS (in-ear-monitor system) for their own hearing sake (with a few more advantages on the stage of course). Marty Garcia, founder and CEO of Future Sonics, is probably the first to complete the system and started the movement of high quality custom molded earphone for stage monitoring. A few years later in 1991, Etymotic answered the call of a more convenient, more universal and accurate monitor that is designed toward off-stage and general music listening, thus born the original ER4 – better known now as ER4B. B is for ‘binaural’, as ER4B is tuned toward binaural recording playback. As good as it is, ER4B is found to be a little too bright for the general, non-binaural recording.  So Etymotic retuned the 4B and came out with the ER4S. S is for ‘stereo’, as in your typical 2 channels a.k.a. stereo recording in cassette and CD. Years later when portable audio became mainstream, Etymotic further tuned the 4S to 4P (as in ‘portable’) by reducing the impedance from 100ohm to 27ohm. Though 4P is easier to drive directly from a portable source such as portable CD player or DAP (digital audio player, or ‘mp3 player’ if you like), it doesn’t sound quite as accurate (a discussion later on). That being said, the only difference between 4S and 4P is on the ‘pod’ of the cable where the resistors are housed inside. By simply adding a 75ohm resistor adapter (which Etymotic does sell), you can essentially change the 4P to 4S. Unfortunately you can’t do the same to 4B since it has an extra pair of capacitors in the pod. The earpieces (where the balanced armature drivers housed) are essentially identical between the three models.