Unfortunately I don't have access to an oscilloscope. I mean, sure it's possible that there is something going on there. But at the same time, I've got Sennheiser HD595's connected to the same computer and those are $180 headphones. You would think that they are more sensitive than the S4's, but I don't know.
Anyway I mailed the support earlier today but have yet to hear from them. I have to agree with Amy though. It feels like something must be wrong with mine. I can't find anyone else with this problem and I highly doubt that everybody is using the S4's with only apple products hehe.