I have the USA spec ipod/iphone adaptor,(2009 version from Science of Speed) works just fine with the iphone 4.
Pandora, Ipod, no problems. It does pop up a message when you connect it saying " this accessory is not made to work with this phone" or something similar but it still works.
In fact. I got the same message with the 3GS iphone but worked then as well.