Drawing boxes to specific sizes - photoshop help

[my_computer]

Hi. I'm utterly useless at most things Photoshop. So I was hoping someone could help me out with something I'm sure is very, very simple

I have an image. Underneath I would like to draw a rectangle of the same size, and then split this rectangle up into a further three of four sections of equal size.

How would I go about drawing the equal size boxes underneath, and then selecting only the needed parts to apply colour. Here is an example of what I am trying to do.

http://goo.gl/pxuSG

Thanks