You should be able to print all 4 on 1 page.
Go to File -> Print Setup, and select "Multiple copies per sheet" under "Printing options". You can also make sure that the side and top margins are set to 0, as well as horizontal and vertical gaps. That way none of those will take the space on the paper, preventing you from printing all 4. Lastly, make sure that the paper you selected to print on can fit all 4 postcards.
Make sure to print preview before printing