The 4, 6, 7, and 8 should all fit. However, the early RV-4 tips are quite thin. The later 6 tips seem to be fabricated with more layers of fiberglass.
I have the original 6 tips that came with my wings and I am very happy with them. I added a set of -4 wing tips for some wing tip mods that I have been playing with, and found them to be quite a bit thinner that the -6 tips. It seems that the gel coat is thicker on the -4 tips and exhibit a lot of voids in the edge laminations. It is all fixable...