The equation for a circle is
(x-h)^2 + (y-k)^2 = r^2, where (h, k) are the coordinates of the center of the circle and r is the radius.
So if the center of the circle is on the y axis, h = 0 and the coordinates are (0, k). k can be any y value.
x^2 + (y-k)^2 = 4 for any k value.
ex. if k=0, the equation for the circle is x^2 + y^2 = 4