A note on the PFE. EQing plays a huge role in how it comes across. Their published response graph shows how rough they are on the top end. Interestingly if you have the option to fine tune the response with some level of resolution, you can balance out this region and rather significantly improve the treble sound of the PFE.
A note on the midrange, the PFE with the gray filter and the ER4S/P share the same overall frequency response. The PFE has more energy but is less thick/textured on note. The PFE is more lively, clean, but the ER4S has a sense of more fine details and a significantly better sound stage. I'm more partial to the ER4S if the option was available versus the PFE. I've never used the ER4P, but taking out amp needs, the P will most likely be a better overall package in terms of sound reproduction than the PFE. I will say the PFE is more fun, but it lacks the sound stage and sense of balance and realism that something like the ER4 has.
There are plenty of other IEMs with good midrange that isn't vastly overshadowed by the other frequencies too. The CK10, the RE252, the SE530 if you want to get spendy and really specifically midrange specific. EQing of course works too.
The RE252 has become one of my favorite IEMs I've used. I'd argue it's the best dynamic driver earphone out there, outstanding balance, great sound quality, excellent extension on both ends, and the street price is tough to beat (got mine for $140). As well as I think of the PFE as a product, I think there are some others out there that are simply outpacing it. A number of the dual driver earphones like the CK10 simply cover the spectrum better and offer higher overall sound quality.
I'm really waiting for Phonak to give us a dual driver IEM. It's just that single BA systems lack the breadth. The SA6 was quite good about it. The PFE is pretty good. However, once you step to a dual driver, you get the whole spectrum and simply better coverage without forcing the ends. I'm so glad Sleek is coming out with the SA7 dual driver because the SA6 was so good but just short changed on range. The PFE shares the same fake, just as the ER4 does. I've long waited for Etymotic to develop a dual driver setup too. For a single driver, I think dynamic is just a better route and things like the IE8 and RE252 show what can be done with that format that so far hasn't quite been done with a BA driver.