How much isolation do you need? Does it have to be similar to the ER4 in this regard? I highly recommend JVC FX700 if you won't mind isolation on par with an average over the ear closed back headphone. It works OK for most environments with just a little bit of extra volume applied. I listen to them on the subway, albeit the subway here in Toronto is pretty quiet. It works fine for me. Of course, you won't be able to hear small nuances in your music, but then I don't think that even ER4 isolation level is enough to hear all details in music in a noisy public transportation.
But anyway, the real deal with FX700 is the sound. It's simply phenomenal. Your ER4 will run away in fear. FX700 stomps all over the ER4 in just about everything except clarity and frequency response. FX700 is not a studio monitor tool like ER4, so it's not as flat, but it is far more enjoyable to listen to. Next to it, the ER4 has a very limited dynamic range, frequency extension, detail, soundstage, timbre - pretty much everything. Those who think ER4 is really detailed are fooled by its small dynamic range that causes quieter passages in music to become louder and more audible. FX700 doesn't do that - it has a huge dynamic range with the proper contrast between loud and quiet sounds in the recording. Its wooden housing also gives it a much more natural timbre than what ER4 can muster. FX700 has a similar quick attack to the ER4 when the music calls for it, but it also has a much longer, far more natural decay, whereas the Etys cut sounds short and do not reproduce the full harmonic palette.
Don't get me wrong, I am not an ER4 hater. In fact, it was the best sounding IEM I've heard until I tried the FX700. Then I realized that BAs simply can't compete with a well tuned dynamic driver. I don't think you will find another IEM in this price range with sound close to FX700 in quality, although if you do need great isolation I recommend Ortofon e-Q7 as a nice alternative to the Etys, with a warmer, less analytical sound, but less detail. Another good option may be Hifiman RE262, but it needs an amp to sound best. RE252 is its low impedance easy to drive brother with a more neutral sound that is very similar to the ER4 in overall balance and character. RE252 housing, however, has a really odd shape and may not fit many people's ears properly. The Hifiman products offer more isolation than the FX700 as well, but are quite a step behind in sound quality to my ears although they do compete very well against the ER4.