Related to the column sums. The first column sums to 9, the second to 10, then 11, 12, and for the last column, 13. Two is the required number to get 13.
I guess I have to admit that a pattern of length 4 is more convincing than a pattern of length 2, so I concede that your answer of 2 is better than mine of 7. Row 1 adds to 25 and row 2 adds to 20, so the obvious "pattern" is that the row total decreases by 5 each time!
Oh, well. I didn't even get that far before seeing your hint. :-(