maybe - although it gets really complicated because a lot of radio 4 stuff is repeated either on the same day or throughout the week - plus 4 extra has much of the same stuff repeated a week later (or years later !) or has an omnibus edition on a weekend. Also part of the idea of iplayer is that you can browse for things you don't even know you've missed.
I think what I'm saying is that I'm not sure how slick a voice controlled iplayer radio would be - obviously they could make you select the program via the alexa app and then stream from the echo, but this also would be a little clumsy. Don't get me wrong, I use iplayer radio all the time and it would be great to have something on the Echo, I'm just not convinced it would be as satisfying as on screen based device.
I'd extend the same argument to why it doesn't work well for shopping either IMO. "Alexa add potatoes to my shopping basket"....."Do you mean 1kg of King Edwards or 2kg of Maris Piper...etc etc "
When voice AI get's more advanced (very soon I suspect) then you'll be able to have a proper conversation with it and describe exactly what you're after but at this point Alexa isn't capable of turning on a light with 100% reliability. "I'm sorry I can't find a device called lates" so the chance of getting last weekends Gardeners' Question Time correctly is slim