The Curator

The curator of the National Gallery in London was a mysterious individual who had insight into a particular Time Lord painting.

After the Eleventh Doctor remembered his face (presumably due to his resemblance to the Fourth Doctor), the curator told him that, "in years to come, you might find yourself… revisiting a few. But just the old favourites, eh?"

He spoke with the Eleventh Doctor and told him that the name of the picture was neither No More nor Gallifrey Falls but in fact Gallifrey Falls No More, and pointed the Doctor in the direction of a search for Gallifrey, telling him he had "a lot to do."

The man then suggested that he had perhaps been the Doctor once, or that the Eleventh Doctor had once been him. (TV: The Day of the Doctor)

Behind the scenes

 * The script never directly states that this character is an incarnation of the Doctor, but it implies that he may be a future version of the Doctor who has changed back into the form of his fourth incarnation, or is in fact, said previous incarnation. Further fuelling confusion is the fact that Tom Baker is not separately credited for this performance, receiving only a credit for playing "The Doctor" (as did all the other past Doctor actors, who appeared in the episode through the use of archive footage) while another evidence of this being the Eleventh Doctor giving a wink to the curator possibly to suggest he knows this man is actually the Fourth Doctor.
 * Early in the episode Queen Elisabeth I's letter appoints the Doctor as "curator of the Undergallery." This is the only other hint that the curator may, in fact, be a future incarnation of the Doctor.