As I mentioned in a previous thread, I have personally had serious problems with the custom earmolds, and now just use the foamies. Basically, if the molds are not made perfectly (and how is the buyer to know such things, even if they are order from one of the recommended vendors), the process of keeping the stems properly inserted in the molds can snap the ER-4s' stem. Etymotic graciously repaired mine twice (out of warranty, I might add), and even installed these neat little tubular extensions, in an attempt to minimize the problem. But the stems snapped again. So now, not to become overly dependant of the good will of the company, I use just the foamies -- which frankly, work better if you're planning on walking the streets of a major city, and look far less dorky! With the foamies, you can do the "cord over the back of the ear" trick, and minimize the microphonics of the 4S cord. I found doing that with the custom earmolds quite difficult.