Michael J. Fox has been fighting with Parkinson’s disease for many years but still has starred in small roles or voice-over animated characters. So, why not another sequel of Back to the Future 4. If the movie does decide to run, he has requested the role of the estranged professor and not stars his usual role in the last three Back to the Future films. Fox is now over forty and feels directors of the sequel need to cast younger kids for the other parts. Most likely the movie will not go into production after this request.

Source: Fark.com