Welcome to the Apple Discussions. You can't. The standard print sizes are 4x6, 5x7 and 8x10. None of which are the same ratio. Even the older 3.5 x 5 is the wrong ratio. You can print them your self on a larger piece of paper and the trim off the excess. See iPhoto: About aspect ratio and print orders.
You might find a custom lab online that will print them up to the 4:3 ratio. Don't know what size they would be, 6 x 4.5 maybe. Try Googling for it.
Hope this has been of some help. Good luck.
OT