Filling in exposed rib cavities
John and the "Bakersfield Bunch" began doing this upgrade way back in the '80's on the early -4's that were being built there. (Maybe John did it on his early -3's and Rocket I, but I don't remember.)
They used expandable foam to fill the cavity, sanded it flat, filled the exposed face with featherfill (or some other light "bondo"), and then finish paint over that. Some builders would use a very light fiberglass to seal the surface instead of bondo, but I don't think John did.
My Rocket is S/N 002 and I chose not to do the mod because of my concern of the potential for water intrusion (especially at the back) and the resultant corrosion opportunity.
Some of the builders would also fill in the ends of the flaps and ailerons.
There are also some "speed mods" out there that actually modify the shape of the inboard ends of the elevators to give better aerodynamics by reducing drag = more speed. Dave Anders has recently completed such a mod on his very fast -4 and believes there is some advantage from the result.
YMMV
David Howe