The parameters of your box will be dispayed ONLY if you still have it selected. Your box should now just be represented by lines. In the modify panel you will see the readouts for lenght, width, height and selectors for segmentation. Set the segments 4,3,4 length, width, height. However, if you want to work ina purely proceedural way, you may want to just slap the modifier on it.
Lets get down to business.
First of all we want to delete all of the polygons on the side of the mesh that we will be mirroring to. We are only going to model half the head to begin with. When active, this mode allows you to manipulate any part of the box in any way needed. Rotate your box in the viewport until you are looking at the left hand side. Yor box should now lok like the one to your left.