Any change made in the first coordinate, is a change of the point in the horizontal direction.
Similarly, any change made in the second coordinate, is a change of the point in the vertical direction.
Thus the rule (a, b)→(a, b-4) is a rule which affects the point only in the vertical position.
The point (a, b) is shifted 4 units down (because of -4)
Answer: a translation of 4 units down
