A family of Microsoft word processing software products for creating web, email, and print documents.
The Avery 5388 label format is the only index card I've seen in Word (although there may be one among the formats offered by other vendors). This format prints only three cards on a portrait-oriented letter-size page. Since you aren't printing on actual index card stock, though, you can easily make a template of your own that puts four cards on a page, saving you money.
To do this, start with a blank document, set its orientation to landscape, and select the narrow margins (0.5" on all sides).
Insert a 2x2 table. Select the entire table, go to the Table Tools > Layout ribbon, and set the cell size to 3" high and 5" wide.
In the empty paragraph below the table, press Ctrl+Enter to insert a page break.
Select the entire first page and copy it to the clipboard. Go to the top of the second page and paste; paste again multiple times until you have as many pages as you'll want to print cards on.
Save the result as a template (*.dotx) file. You can use that template to start new documents of the same set of tables.
One additional advantage of this over the Avery format is that the borders of the table show you where to cut the paper to fit on the index cards.