It wouldn't be beneficial, and it wouldn't destroy the ER4. What the resistor tends to do is increase sound stage, treble, and detail. Too much impedance will exaggerate those features to way too far a degree and the IEM will no longer be enjoyable. I have a 100ohm resistor that I used with an HF3 (it gives similar effects to using it with an ER4P) and the difference was noticeable compared to the stock Etymotic resistor which is 75ohm.
I... I have a question. People keep saying that when ER4P transformed into the S and fed with enough power, it will sound better. The question is, if I use... say... 300 ohm impedance adapter with my ER4P and feed it with enough power from my Xonar STX sound card, would it be beneficial or would it destroy my ER4P? The Xonar can support up to 600 ohm of gain.