It should be 12 ft away from the wall. Not 4 ft. You do not just subtract the difference because as you pull it away from the wall, the ladder is mostly moving horizontally with only a small vertical change.
You can use Pythagorean Theorem to find the answer. Think of the ladder as the hypotenuse of a right triangle with the wall and the ground as the legs.
a^2 + b^2 = c^2
16^2 + b^2 = 20^2
b^2 = 20^2 - 16^2
b^2 = 144
b = 12 ft
