I think I have a solution, I'll buy the ER4 PT's I think I can get them for $200, then I send them to etymotic and have them convert them to ER4s that will ensure that I get a matched pair and the ER4s I wanted, question is how much would it cost to convert, I don't want to spend more than $250
IIRC, Etymotic doesn't 'upgrade' one ER4 to another. If you sent in a 4P, you will get a 4P back (for warranty). I do remember they refused a person's similar request last time and they don't sell the cable for different models out. 4PT is essentially just 4P, and 4S is 4P with added resistance (100ohm vs 27ohm). The difference between 4P and 4S is in the cable's pod (which houses metal resistor inside). The drivers themselves are identical between the two models. Even the 4B is using the same driver (the difference is also in the cable pod, which has a pair of small capacitor beside the 100ohm resistors).
I want the ER4s because I want my ultraportable rig to be small as possible and I don't want to use an adapter. but I think you might be right, I remember reading somewhere that there is something in the ER4s driver that makes them unique, but I need to bug them to see if it is possible. If not, I think I might buy the westone CR1 custom IEM's... I dunno, $250's the limit.
ebay adapter isn't that big anyway. Much cheaper and smaller than the one Ety sells.