Fred, I don't have any info on how to "clean, repair, and improve the over all quality of old 4 x 5 cellulose negatives?", but can give you a reference regarding how to continue to store them, whether they are cellulose nitrate or not. If you continue to store them, they're always available for whatever duplication methods become available in the future.(posted 9006 days ago)In Henry Wilhem's comprehensive (encyclopedic?) book (1993) "The Permanence and Care of Color Photographs: ", ISBN 0-911515-00-3, Appendix 19.1 (12 big pages including references) is titled, "Freezer Storage for Permanent Preservation of Cellulose Nitrate Still-Camera Negatives and Motion Pictures".
This chapter discusses characteristics, history and identification of cellulose nitrate films. The stages and mechanisms of deterioration are discussed, as well as some literature references on salvaging and restoring old negs (page 686, reference #12).
Two of my interpretations from this chapter are that 1) fire hazards of small quantities of still camera negs are greatly overrated and 2) cold storage at lower RH DRASTICALLY prolongs the life of cellulose nitrate. Even though it's pretty dry reading, I can't hardly praise this book enough. If it's not in your local library, Amazon lists it at only about $40.