From what I've read about fiberglassing, it adds quite a bit of weight to the finished plane. I'm not sure off the top of my head what the wing loading is on the RV-4, but you will most definitely change it by 'glassing the airframe. I'm in the process of covering mine right now. If you do enough sanding, it actually covers pretty well. You can see the ribs on mine, but just barely.
chad