This is a very interesting thread!
If my child had asked me, I would say that the actor playing Mickey Mouse can't talk through his mouse costume. What illusion woud I be breaking? Does your 4 year old really believe it is a giant talking mouse? Does a 4 year old notice the difference between cartoons and live TV?
I've alway pointed out the difference to my kids from the youngest age the there is a difference between the character and the actor. I mean, don't your kids participate in play acting/ children's theatre in pre-school? When my daughter was 4, her pre-school sang songs from Annie. "It's a Hard Knock Life." The kids knew they were not orphans, only acting like ones.
It seems to me the important ilusion to maintain in this case, is that there is only one Mickey Mouse. THAT is what makes the experience so special.
As for Santa, though, I would never tell my children that he is not real. After all, I'm still working on that one myself.