Unfortunately, no. You can use the GeometryService task to buffer a point by 400 m (or whatever distance you want). This will yield a polygon that looks like a circle.
If you don't want to use the GeometryService, you can use a little trigonometry to find the points along the circle's circumference.
Let r be the radius of the circle. Let (a,b) be the center of the circle. Then to find points (x,y) along the circle :
x = a + r*cos(theta)
y = a + r*sin(theta)
Vary theta from 0 - 360. Use small increments to get a more detailed circle. Create a polygon with a single ring that contains all these (x,y) pairs. You have a circle.
