how do I divide a page into 4 boxes in order to create small notes to put into greeting cards?

First, I would visit Pages Preferences : Rulers panel. Select the following, where any guide color is of your own choosing:

Back in your blank document, you insert a text box. Drag it to the upper left corner as it will become your first quadrant. As you drag it to the left, an automatic alignment guide will show the left edge of your document. Now drag the right grip of the text box to the right until you meet with the center document alignment guide. And expand the box downward until it encounters the center alignment guide. Your first quadrant is done unless you want to add a thin border around it.

Pressing the option key, drag the first quadrant directly to the right until it just crosses the automatic vertical center alignment guide. Two quadrants done.

Press the option key and drag the first quadrant box straight down just below the center alignment guide. Repeat this with the other 2 quadrant box and you now have all four quandrants present in your document.

You are welcome.

You have specific, easy to understand and helpful instructions. Thank you!