The only plausible plotline for BTTF4 would be for Marty to travel into the future to a time when Parkinson's is cured. Michael J. Fox hasn't been seen much publicly for a reason: the guy is suffering. As far as rumors go, this is a bad one, probably resulting from some line dropped by an idiot first-year marketing exec at Universal in a brainstorming session. Milk that udder till it's dry and then lick off the drops.