How can I divide my page into four even parts for flash cards?

The easiest way is to use a "label" definition designed for four-up cards.

1. Select the Mailings tab in Word.
2. In the Create group, click Labels.
3. On the Labels tab of the Envelopes and Labels dialog, click Options.
4. If you are using commercial stock, such as Avery note cards or postcards, you can choose the appropriate Avery stock number and click OK.
5. If not, select Microsoft as the vendor and select one of the appropriate 1/4 Letter options (one is landscape and one portrait) and click OK.
6. Back in the Labels dialog, click New Document.
7. This will give you a page divided into four quarters. It's set up as a table, so make sure you have table gridlines displayed so that you can see the card boundaries. When you click anywhere in the table, the contextual Table Tools tab will be displayed. On the Table Tools | Layout tab, make sure that View Gridlines is turned on.
8. By default, you get one divided page (a 2x2 table) and one runover page (with just a single empty paragraph. As you finish the fourth card and press Tab, a new row will be created (as in any table).

The downside to this particular arrangement is that, unlike with other labels that have a bottom margin, you will always have an extra blank page because of the empty paragraph that Word requires after a table. Formatting it as very small (a typical technique) will not work here, but formatting it as Hidden will (see http://sbarnhill.mvps.org/WordFAQs/BlankPage.htm).