I doubt he only knew 4 knots. Looking at online photos, is difficult, but in this photo
I think I can see a double fisherman's on the cord on the back of his harness, a figure eight on a bight on the blue haul line, a girth hitch on the last pin and possibly a water knot. I have also found pictures of him at an anchor with a bunch of clove hitches, but it is unclear who tied them. Maybe he never used them, but I doubt that.
I would also expect him to know the Prussik, or some other friction knot for hauling and a Munter hitch for belaying, but cannot find them in photos. He probably also knew a square knot (and hence a granny knot) as well as a simple overhand knot for backing up his eight on a bight, because most people know these knots. Finally, even if he used a swami belt, I would be surprised if he couldn't tie a bowline on a coil.
This is still a relatively small number of knots.