If you want full control of the iPhone4 I recommend the DICE Mediabridge. You need to post your year/model X5 so we may determine if you have digital or analog DSP. If it is digital DSP you will need a converter for an additional 80 USD. I have the Mediabridge, and I don't use the iPhone4 but it works very well with my Android phone.

-Cyrus

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2002 BMW X5 4.4i