In "The Shakespeare Code", the Doctor refered to the Harry Potter books, even mentioning "good old J.K. [Rowling]" by name. It is not made clear if the films also existed in the Whoniverse. However, there is nothing preventing the fourth Harry Potter from being created in the world of Doctor Who with David Tennant. There have been other people who have looked like the Doctor confirmed to have existed in the Doctor Who Universe, notably Ramón Slamander ("The Enemy of the World") who looked like the Second Doctor, Maxil who looked like the Sixth Doctor ("Arc of Infinity") and Caecilius who looked like the Twelfth ("The Fires of Pompeii").
Plus, the Harry Potter films could easily have existed without David Tennant. Maybe they got a different actor.
And, even if the films were never made in the Doctor Who universe, there's no reason why the books shouldn't have been.