Well, instead of going with either of those two, I decided to bite the bullet and get the HF5 (I realized deep down that I would regret not getting them since ive been wondering for years how they sound). My immediate impression (after I took the small triple flanges off and put on a pair of the trusty grey ones) was that the body of the headphone was shorter, and that sadly I could not just insert them the same was as I do with the ER4. Apparently I insert the ER4 even deeper than this:
http://www.head-fi.org/t/649046/hd600-in-the-form-of-an-iem-with-very-good-isolation/60#post_9305716 , and thus to get the same type of fit with the HF5 that I would have to stuff them so far in my ears that I would soon damage them since I had to pull them out by the cord. It didnt take long for me to figure out that I could still get a good seal without inserting them as far as I do with my ER4 (im definitely getting a great seal though), but this is something that I hadnt even read about when people were comparing the two side by side. Honestly it surprised me how different the fit was, since I was just expecting it to be the same.
Since it took me a couple weeks to get used to the fit of my ER4, I was incapable of deciding whether they needed some break-in, but im not even going to specifically ask the question since it is controversial. Feel free to share your opinions nonetheless on whether I should expect them to sound differently after ~100 hours of use, since this may or may not skew my first impressions on the sound.
I have only been messing around with them for a couple hours, so this is going to be my preliminary first impression. The build quality is noticeably less robust than the ER4, but there are a few things that I actually like about it. First off, the cable length is much more manageable, and I dont have to coil the cord in my pocket (which on the ER4 eventually tangles the cable since I take my iPod/cell phone in and out of my pocket). Also the thinner, softer, and lighter cable is a nice addition and more comfortable, though without a doubt im worried about it's durability over time. I also like the rubberized coating on the body of the IEM itself - this literally makes the headphone softer and more comfortable in the ear. And as for the 45 degree angle connector, I actually like it better since imo it doesnt stress the jack as much when moving it around, but I imagine that this is debatable (my ER4S extension has the same 45 degree angle). So overall the build quality seems to be substantially less durable, but also results in them being more comfortable to wear. It reminds me of my cheap JVC marshmallows that I used to own, except a little more robust. Since I bought these despite already knowing about the build quality difference, I am ok with it, but im just making it known that these are not built like the ER4. The non-removable cable will probably result in these only lasting a couple years at best of regular use for me (im already worrying about cable snags).
Now the reason why I bought the HF5 (other than being an ER4 backup): the sound. Note that any sound comparison will be with the red filters installed in my ER4 (of which im already thinking about swapping into the HF5), and most likely in the P configuration since volumes are more comparable that way. There is definitely more bass presence with the HF5. Since I own the ER4 you can imagine that im not a basshead, but I feel like the HF5 provides the slight extra punch that the ER4 just does not have. This can of course be altered with EQ (I dont find this to be a basshead IEM or anything), but stock the difference is definitely noticeable (probably like a +3db boost in the upper bass range). Im not sure if it is just because of the extra bass or not, but I find the HF5 to be significantly warmer in sound, but there is also that boost in the upper midrange that I remember was the reason why I ended up using the red filters in the ER4. The HF5 is very precise and I can hear a lot of detail, but as expected it seems veiled in comparison with the ER4. It just does not have that straight up clarity that ive been used to, but as a result the sound is definitely warmer and slightly more musical. The veiled sound also gives me the impression of a slightly smaller soundstage.
Overall, im pretty satisfied with the HF5s so far. They seem to be about as close to the ER4 as I was expecting without getting another pair of ER4s. I am actually hoping for the veiling/slight muffling of the sound to change as they break in, but they still sound really good. I still find that I like the ER4 significantly more, but more time getting used to their sound may eventually close the gap a little bit, since ive found that my initial impressions of headphones and IEMs tend to change a bit over time. As a backup IEM to the ER4, I am completely satisfied with them at the moment. I will miss my ER4s while waiting for the new cable, but the HF5 sounds good enough that I wont exactly be living in despair.
edit: was just messing around and decided to see if the ER4P to S converter would work with the HF5, and it didnt. I guess the wiring doesnt match for some reason. If I were to pull the converter cable out of the source a little, the sound would be clear, but I didnt mess with it too much since I didnt want to do any damage to the headphones, and it was clear that it didnt work. At the very least I expected them to be compatible, very strange.