Good suggestion, Tom. I also would like to keep my 4 as original as possible. I installed a 21FB in a CTC-7 once, and used a Cool Whip top cut in half with GC high voltage putty between the halves. I wedged it between the ring and the CRT with great results, no arcing. I know Cool Whip is still available, but I don't know about the putty. I'm pretty sure a sub could be found to work.
Kevin