Radio 4 does not need to seek younger listeners because listeners, of any station, are maturing so they will come to it in due time. What matters with all of the BBC's output is that it keep pace with the world in which it broadcasts, reflecting our culture, technology and tastes without pandering outside of its remit.
Radio 1 is for young listeners, whether by age or inclination. Radio 2 is for the middle range and ditto. Radio 3 is for lovers of classical music (and IMHO should not be dithering about with semi-classical works except as 'items of interest'). And Radio 4 is now, always has been and evermore be a mature, considered and intellectually stimulating source of news, current affairs and general information along with quality drama; a station to which we all turn in time. And that time will come whatever age we are and in whatever age we are living. What it must never do is alienate this mature audience by pretending it is in any way Radio 1 or 2, or even 3 to a degree. It is all the BBC; they are all BBC listeners.
Soon after I read this news item I heard the absolutely perfect example of what Radio 4 is about - The Media Show. The excellent Steve Hewlett included an interview with a Talk Radio executive. It was his interpretation of 'Talk Radio' that brilliantly explained why Radio 4 is so good. It is NOT talk radio, as he clearly understands. It is speech radio. He seemed also to understand that even in the sports arena it was not where his kind of radio would go. Maybe he'd make a better trustee than those wanting change for change sake.
The trustees have NO role in determining the style, format or output of any individual station. They are about the BRAND, which is The BBC and the standards, ethics and finance of its component parts - radio stations, television stations and now new media (which is narrowcast, not broadcast, really but that's another story).
The executives in charge of the stations and their output need to be very careful of listening to any siren voices, even among their apparent allies. We have seen before serious tinkering with the format of Radio 4 had to be largely rowed back as audiences declined. Evolve by all means. Stay modern. But serve the mostly 40-70somethings you have always served since being spawned out of the Home Service and Long Wave all those years ago. That audience died long ago (well a few of us live on) but it has been steadily replaced with new ears attached to more modern brains. Keep up at the back but please don't wander off in search of some young flesh. You might be middle aged now but having a mid-life crisis will not help anyone.