With the Fifty Year Anniversary of Doctor Who coming in 2013 there's a distinct possibility that they would invite all of the surviving Doctors to appear. If they do such an episode would you appear as the Fourth Doctor again? If they can explain the sudden aging Peter Davison suffered in the programme surely they could do the same for you. (I don't think the production team would ever consider such a episode without your involvement.)
Best wishes from America,
Cody T.