Give Total Seal a call. They usually have stuff when all else fails, but you end up paying for it.
I don't show anything in my catalogs either. The ring companys are really lacking in the 4.200(ish) bore area, when talking specialized rings.... it's just not very common stuff. I am assuming you want to stay away from a moly faced ring.
I also have a 4.200 bore. I called total seal and perfect circle, neither had a tool steel/hellfire NOS ring. I believe they had moly rings but I also need a NOS ring. I was told I would have to custom order them.
Check with the AMC (yes American Motors) Guys, the 390's and 401's hover in that range. Check with Hemi Atkins or one of these dudes from West Virginia. They are always up to something and they may have what you need in stock.
Call diamond pistons, they have a ringset available for that. Its a stainless steel ringset. .043 x .043 x 3mm for a 4.200 bore. There number is 1-877-552-2112. Mike was the one their that helped me out the last time i called.