Honestly, though, since the minisode Night of..
was filmed in the last two days of production for the Day of..
its reasonable to argue that Tom Baker AND Paul McGann did appear in this. And really, its only fair for McGann, too - poor old Sylv is the only one without a televised multi-Doctor story.
Back on topic... Personally, and after some thought, I really like the idea that this is a splinter of the Fourth Doctor that did leave out his life in retirement as the Curator. And furthermore, I also like to entertain the idea that HE is the infamous Watcher. Would make sense, as they'd be both Fourth Doctors, but the Curator would probably need to "fuse" with the proper Fourth Doctor, to complete the regeneration for both sides.
Its a fan theory, obviously, but really, its a nice one. The idea that somehow the Fourth Doctor and the Eleventh Doctor had a scene together just fills me with glee.