Hmm. Interesting. What you're saying makes sense; there is sign that the quarter wavelength resonance frequency is moved down in the (P)R measurement, pointing to less insertion than the ER4P, it looks like there's more extra 10-12 kHz resonance of the 711 coupler. The 4-5k bump looks like the switch from plastic to PVC acoustic tubing. I see that in most reshelled ER4 models. Don't know how it's audible. A tapered horn would probably help the PR interface. Well, actually making it custom-fit would probably help more.
I still can't get over how you guys can be okay with the comfort of the ER4. My ears are constantly sore any time after using them. I don't like foam tips or gliders.