What does "4 up" mean?

4UP is a printing term that means 4 per page.

Burris Four-of-a-Kind Postcards™are 11 in. X 8.5 in. blank sheets of postcard thickness paper that are designed for printing on standard laser and inkjet printers. They have a special micro perforation so each sheet will yield four 5.5" X 4.25" postcards