In which case, you are right: acceleration
and its magnitude
To find the direction, just think about the vector : this vector is units in the direction (i.e. 4 units to the left) and units in the direction (i.e. 2 units downward.)
So, draw a right-angled triangle having one side horizontal and 4 units long, and at its left-hand end a side 2 units long vertically downwards. Join the hypotenuse: this side represents the actual acceleration vector. See the attached diagram.
So if the acceleration makes an angle below the horizontal, .