"after spending the last four seasons in the titular role of The Doctor" - no, it was only three. The British do TV different than the Americans but there were only three "seasons" (including the current one) with Smith.
"where he will star alongside a majority of the other actors who have taken on the character" - That was the fan theory ages ago, but the casting has long since been confirmed by the BBC and David Tennant is the only other former doctor to appear in the special.
Regardless, Smith had a great run. I was skeptical at first at the "youngest ever doctor" but I was thrilled with the result.