The Back to the Future films are my favourite films.
But I do not want a part 4. It would seem really out of place alongside the first three, considering the first three fit together perfectly and conclude perfectly, and Michael J Fox is obviously a lot older now so the film could not 'pick up where it left off'.
However...
I would obviously love more Back to the Future, so would not be against the idea of a new movie or trilogy that was more distinct and independent from the first trilogy, that didn't interfere with the events of the first trilogy, and with a mostly different cast to set it apart further.
For example, something like a new Trilogy called "Return to the Future" featuring Christopher Lloyd as Doc Brown and maybe some new characters such as Jules, Verne, or one of Marty's kiRAB of the "successful Marty" timeline, with maybe a cameo from Marty. It could be set in the further future (2045 or something) and they could go back to the 1980s perhaps, but not mess with the first film.
It is kinda perfect that Christopher Lloyd's Doc was always made up to look old, as he could easily resume the role without anyone thinking he looked very different.