There is no indication that fans are ready for a replacement character for the fourth film, however, that does not mean that it will not happen and be good. Downey, Jr is not the casting director and has no say in whether he will be able to take on the lead role one more time, so only time will tell at this point.
The release date for a movie like Iron Man 4 at this point would not be for a few years, so there is plenty of time to get the cast together, with or without Downey, Jr.
Marvel is on a role with new movies coming out the next few years. At least one of them will have Iron Man in it, while the others like Captain America: Civil War, Doctor Strange and Guardians of the Galaxy 2 will all be without the help of Iron Man. None of those movie is set to arrive before 2016.