I haven't spoken to anybody at Riley's directly - but talked to one of the dealer's they list.
According to him - Riley's owns the machinery to make the 4 1/2 pistons, but stopped making them about 10 years ago. They have sold all their stock.
So - like me, you're out of luck unless you can find some used or old stock somewhere.
I guess it's either bore to 4 11/16 and deal with the associated issues there - or sleeve.
Guy I talked to recommended using the original pistons with larger rings (ie. 4.5 + .045) - machining the pistons to fit them... I'll have to talk to my machinist about that. I've never heard of anybody doing it, but who knows.