Hi Arckivio,
When I installed a smaller 4-jaw independent chuck on my small lathe I already had a backplate with the correct register diameter, but with 3 mounting holes. So I simply put a mounting bolt in one in one of the holes in the backplate and used one of the 4 holes in the 4-jaw, then I could use the backplate as a drilling jig to mark the position of two new holes at the back of the chuck and drill and tap those two holes. Saved me some work and time, I agree 4 bolts would have been better but a couple of the holes would have interfered with the bolts that mount the backplate to the lathe mandrel. It has worked well for several years.
Thor