A particle moves on a circle of radius 4 cm, centered at the origin, in the $xy$-plane ($x$ and $y$ measured in centimeters). At time $t=0$ it starts at the point $(0,4)$ and moves counterclockwise as $t$ increases, going once around the circle in 9 seconds at constant speed.

(a) Write a parameterization for the particle's motion.
$x(t) =$ ,
$y(t) =$

(b) What is the particle's speed?
speed =
(Give units.)