They seem to have started keeping some programmes available for longer, which is great. But the logic of it all escapes me. I've been catching up on ISIHAC: at the weekend they had episodes 5 and 6 available only for another 2 days, but 10 days for episode 3. However, episode 6 is still there and apparently available for another 6 days, as is 3. So why would the older episode be available for longer? And what happened to no. 4?
I think part of the problem/ confusion is that some of the comedy broadcast on R4 us given a fast replay/ rerun on R4 Extra about 2 weeks later, so when iPlayer shows which episodes are available, you get this strange set of results.
I am guessing that when you posted 5 and 6 had just been on R4 and 3, had been on R4 Extrabut episode 4 had not yet been played on R4 Extra hence the situation you described.