I went back to find a prior External Selection today, and discovered that PL 4 is reporting 0 files for recently created selections. (Older selections from October 30 and before mostly but not exclusively show properly.) The good news is that if I click on a specific selection, the files I had selected reappear and the count goes from 0 to the appropriate number of images for that selection.
Looks like a bug to me. Annoying but not fatal.