What paper path are you using? I'd try the front loading manual feed path which keeps the paper pretty flat.
The paper is 13 mil which is 0.33mm thick, so setting it to 4 should be enough. But all the suggestions are worth experimenting with, paper thickness to 5, platen to widest and adding some drying time. Most of these recent PK ink art papers seem to be pretty delicate. the bigger printers use vacuum to keep the paper under control so they tend to handle these papers a little better.
Moving the head from 4 to 5mm would increase the distance the ink travels so theoretically it could affect sharpness, but I would be surprised if you could see it even with a loupe. Platen gap helps keep the paper flat and could easily be causing the small scratches. With sheet paper I don't think the platen gap would have any affect on sharpness.
All ideas worth trying.