I'm using the "Keywords" field.

Here's what I've learned, and it is very strange:
I use IrfanView to edit my IPTC data. It works for all sites (except VMP), as well as DeepMeta, and Windows Explorer (i.e., the keywords show up in the "Tags" column when viewing the directory).
However, loading a file into PS4 and looking at "File | File Info", the IPTC tab is empty except for fields populated in Bridge. In Bridge, I see these, but I also see the title and description (but still no keywords other than the few I add in Bridge).
I can't understand what is going on, but it is consistent. I've tried numerous files, and they all act similarly.