The R4's have gotten a reputation for being poor rebuild candidates, but I believe that this depends on the rebuilder. I remember reading some time ago that the guts are available as an assembly, so it's a matter of proper techniques. I don't what this entails, but I do believe that there are indeed some outfits that can do a good job of rebuliding them.

Unfortunately I have never used one from a competent rebuilder. I've replaced a few with rebuilts and never had good luck.

My $0.02