Thanks Tjibbe. I tried deleting the only piece of software that had changed since I installed the episodes 4 and 5 (the sometimes praised, often reviled MacKeeper), but it made no difference...
At least, after deleting and being unable to re-install episodes 4 and 5, I was able to recover them from a Time Machine backup.
I am a community moderator, here to assist, keep the peace, and keep these boards clean and friendly for all. I am a volunteer, and not a Telltale employee, and my opinions are my own.