Youtube comments are Youtube comments.
If Baker, or any previous Doctor besides Tennant [spoiler]and Hurt[/spoiler] is going to be in the Fiftieth Anniversary special, they managed to get him to a shoot without anybody noticing, which seems pretty unlikely given how the entire British entertainment press was on the lookout for that exact thing. That doesn't rule out the possibility of more previous Doctors showing up throughout next season, which could still happen and which I'd love to see, but there's also nothing to substantiate that theory beyond fanboys really wanting it to be true.
On the other hand, a teaser like this suggests something more substantial than just hawking merchandise or upcoming DVD sets. It's nothing to do with the alleged found episodes because Baker's not in any of them, and it's not related to An Adventure in Space and Time because that's about the beginning of the show and apparently focuses almost entirely on the Hartnell era, with Troughton briefly showing up in the ending.
And speaking of Hartnell and Troughton, well, clearly the #thefourth hashtag suggests we might be seeing more teases with more past Doctors -- so why start at Four?
Well, it could be because he had the longest run and is probably the best-known of the original seven. But it could also be because he's the earliest Doctor who's still alive.
I DO think this is probably leading up to some announcement about past Doctors reuniting. Maybe not on the show; maybe it'll be a retrospective or a convention or something. It'll be interesting to see.
