ok so files are stored in your iphone 4 (hopefully not mp3 but in apple lossless format for sound quality), then it goes to the dac and then amp of your iphone 4 and then to whatever playback device. for amps, after you get it, you will have to get a lineout cable that connects from the iphone 4 to the amp, so it bypasses the internal iphone amp and uses the external amp. the iphone dac should be decent for now though you can bypass the iphone's dac as well by getting the fostex hpp1 (dac/amp in 1 unit), cypher labs algorithm solos (dac only, you will need an external amp besides the solos), the upcoming centrance m8 (dac/amp in 1 unit), go dap x (dac/amp in 1 unit, i heard you can also customize the go dap x's amp section), sony pha1 (dac/amp in 1 unit). those are all portable or as least transportable, and also pretty pricy. there is the cheaper hrt istreamer if you want to bypass the iphone dac but it wont be portable. for home use, its probably better to use a comp for music because idevices only have so many choices to choose from. so back to your situation now, i would say, stick with your iphone dac, get a cheap amp from fiio or something, and then get the headphone you end up picking. if you want later, you can consider one of those dac/amps if you want high quality music for portable use with your iphone. sorry if this is still too confusing, i tried to explain as best as i could and provide information that you may want to know. im willing to explain further if you want more details or clarifications.