simple one

The fair had arrived in town. "A prize for guessing the number of balls under the mugs," shouted one stall owner, as he placed four upturned mugs on the counter in front of him. Each concealed the same number of balls and on each mug was a statement about the number of balls underneath : One or four; Two or four; Two or three; One or two. Only one of the statements was correct. How many balls were under a mug and which statement is true?