Answer
$(0, -4)$
Work Step by Step
RECALL:
The point $(x, y)$ has:
$x$= directed distance of the point from the y-axis (positive when to the right of the y-axis, negative when to the left)
$y$= directed distance of the point from the x-axis (positive when above the x-axis, negative when below)
Point B is:
on the y-axis so $x=0$
4 units below the x-axis so $y=-4$
Thus, the coordinates of $B$ are $(0, -4)$.