Hmmm... This got posted to the official Doctor Who youtube channel last Thursday (on the 4th). Possibly it's a plug for a forthcoming DVD set (which is a bit odd, given that the only 4th Doctor story yet to be released is Terror of the Zygons), or maybe they're going to release short little teasers for all the Doctors, leading up to the 50th anniversary special. Either way, it'll be interesting to see what comes of it.