Unfortunately I cannot remember when I had my iPhone 4.
My iPhone 4s (with iOS5) can stream Spotify to my speakers using an iPod's USB cable.
Sometimes, I'll have Spotify playing, then I plug in the USB and select it through my non-iDrive system. At this time, the music sometimes stops as my phone's Music app opens and begins to play. I have to manually go back to Spotify and start the song again. After that, it continues to play Spotify. Perhaps it would also help to close the Music app.
2011 328i xDrive - Automatic