If Radio 4 is important I'd recommend Wunder Radio for the iPhone. It's £3.99 but does so much more than FStream and makes listening to radio really easy.

If you want to use FStream, then visit http://bbcstreams.com/ to make sure you are using the correct stream. I had a quick look and se that the radio 4 stream is:

http://www.bbc.co.uk/radio/listen/live/r4.asx

Hope that helps.