I don't remember is PSE 4 has the Automatically Suggest Photo Stacks command, but if it does, see this recipe:
I have PSE 4 on an old laptop so I took a quick look and my conclusion is no v4 does not have Suggest Photo Stacks.
Find with Visual Similarity is in v4 and I did do a Catalog > Recover with the build Similarity Index choice as preparation for using that Find feature. However using Visual Similarity on any signficant number of photos would seem very tedious. Of course it could be done systematically on a few photos each time over an extended number of days - but still tedious.