understanding rects/coordinates in psychtoolbox

Hello,

I am trying to write a program that seperates my screen into four quadrants. A circle will randomly appear at the center of each quadrant and the participant must press the spacebar once it appears.

However, I am unsure how to create four rects that are centered at each quadrant. Could anyone provide some knowledge on how I could calculate coordinates/rects. Thank you so much. Much appreciated.