Taking a 4 by 5 grid, How many possible Knight's tours are there starting in the upper left corner and ending in the lower right? (Reflections and rotations don't count)

(A Knight's move is as in chess, an L shaped move, 2 squares in one direction and 1 square in the other direction.)

A Knight's tour is visiting every square once and only once (and the square you start on is considered "visited" for these problems)