Update: Sadly Radio 4b is no longer on air, as the BBC disabled the XML view of their programme metadata in April 2017. They mention "providing a durable successor" to the interface - if that happens in the future, Radio 4b might return.
Built upon that sinking feeling of tuning into Radio 4 and hearing people acting at you, Radio 4b plays you a stream of random programmes from Radio 4's enormous factual archive. You might get the last thirty seconds of an episode of the Archers, but that's all. You will never hear middle-class actors tapping a teaspoon and talking about divorce.
We automatically filter out You and Yours and Money Box Live, because there's so much of it archived, and Woman's Hour, because it contains a drama segment. If there's anything you want to filter manually - maybe Front Row isn't worth the risk of catching the end of The Archers, and you never want to hear Farming Today - you can add exceptions to the page URL (eg. ?butnot=front+row,farming+today). Similarly, you can use "?butonly=" to stream a single show or keyword, or both at once to define a clearer set. As of April 2012 you can also superimpose pub background noise onto In Our Time.
Radio 4b was built by Kevan Davis in March 2011. It's not affiliated with the BBC.