Asked our Deere dealer about putting RTK on 4640 and he laughed at us.  I don't know if there  was a problem with Deere systems on older stuff or if they only want to sell new.

Went with Trimble and it worked great.  Have one with platform kit and an EZ pilot on Steiger.  Platform kit works as good as any system I've operated and EZ pilot does an OK job.  Don't know if I'd want EZ pilot for laying off rows as it takes longer to aquire the line than I'd like but once on line it works fine.  Trimble is supposedly working on a fix for the problem.