You have the center x0, y0 and the radius r. Basically you need the parametric equation of circle:
x = x0 + r * cos(t)
y = y0 + r * sin(t)
Where t is the angle between a radial segment and normalized x-axis, and you need to divide it up as needed. For example for your four points case you do
360/4 = 90
and so use 0, 90, 180, 270 to get the four points.
