That's an excellent comparison you got there. Having owned HF2 and ER4P/S at the same time I agree with you - HF5 is, warmer, more forgiving and more fun to listen to overall, but ER4S is all about accuracy and has incredible, clarity, definition and transparency, especially in the mids. Personally, i don't like the ER4P very much though - they are less fun than HF series and yet not as transparent and accurate as ER4S either, so pretty hit and miss in my view. HF series are some of the most balanced headphones I ever heard and in a way maybe even more so than ER4S (and definitely more balanced than ER4P). They have some warmth, yet also just the right amount of detail and definition to allow them to walk the fine line lending themselves well to both analytical and musical listening IMO. Every frequency range seems to be present in just the perfect amount with them and I disagree with those who think they lack bass or have too much treble. Also, for those who are not aware - all HF series Etymotics sound pretty much identical.
With this being said, the current crop of dynamic driver IEMs are also pretty darn great sounding and approach balanced armature drivers in clarity and transparency. Out of the dynamics I tried, I highly recommend JVC FXT90 which are a wonderfully balanced, inexpensive dual driver dynamic. It offers much of Etymotic ER4S transparency, but also sounds more powerful in the bass, has better dynamics and a more 3 dimensional soundstage. I think it's detail is as good, or very close to the HF series. Overall, both are superb choices for sub $200 IEMs.