As encouraged by the manufacturer, ER-4P can be easily turned into ER-4S, by simply adding a resistor in series. However, it has been proven quite tricky to turn ER-4P into ER-4B, since ER-4B has a dedicated high-pass filter(100 ohm resistor & 220 nF capacitor in parallel) implemented in its barrel.
Once again, my friend vAsurada, let me borrow his DIY-adapter made for ER-4P, a 3-way switchable passive filter which converts ER-4P into a ER-4S and ER-4B, simply by clicking a switch. He told me (with some doubt) that the accuracy is 99%! And I am pretty sure my regular visitor, Carrot, is the designer of the high-pass filter, but please correct me if I am mistaken.
And here's the actual measurement of unfiltered ER-4, P&S&B superimposed together at 94 dB SPL @ 1 kHz. The data are very accurate, even by comparing them to the data provided by Mr. Don Wilson, the designer of ER-4. (different insertion depth in consideration)
4P to 4B accuracy is the sole target of interest, since 4P to 4S adapter is simply a resistor in addition. Its accuracy is rather insignificant.
And here is vAsurada's adapter:
Wow, the difference is less than 1 dB within 20~20,000 Hz! If you have this adapter in your possession along with ER-4P, it is same as owning 3 different IEMs at once, only for much lower price. I don't know about the percentage of 99%, but the adapter is damn accurate. I should return the adapter soon, and if you have any interest of purchasing one, ask vAsurada, not me. (no commercialism, please) Last time I asked him, the price was whooping 250 in USD! Anyway, it was a very fun experience, tinkering with this adapter. Thnx, vAsurada!