I just picked up a very clean Nikon S4 but I'd like to give it a CLA, any recommendations for a person, preferably in the MidWest USA, who is good with the rangefinder cameras?
I've been watching and see nobody has been able to answer your question, and neither can I given I'm on the other side of the Atlantic. But I think generally speaking if there are no actual mechanical problems with a Nikon just use it. If there are things that are obvious that need dealing with there will be generalist repairers who have a wide range of knowledge and a Rolleiflex is a complicated camera to repair and so is a Nikon rangefinder. And if it's just cleaning a dim viewfinder the haze builds up in the back of the main viewing window, and it's a relatively simple job to unscrew the facia and clean the windows yourself (guides are available online). I'll refer you to the extensive knowledge of Richard Haw with Nikon rangefinder cameras https://richardhaw.com/
I'd second Don Goldberg (aka "DAG"). I had my Nikon SP serviced by him last year. It took longer than he stated it would take, but the work was perfect and the price was reasonable.
Crystal Camera in Asheville, NC. My S2 had numerous issues and after a drop the RF was way out of wack. Fixed it up as good as new for less than $200. They have also done my Nikon F2 and meter, my Minolta CLE, and a few other cameras.
Just an FYI, I contacted "DAG" this week about my S4 and he is really booked at the moment and only accepting Leica repairs for the time being. He told me to contact him in 3 months or so.