I hate this kind of puzzles, since the solution always feels kind of arbitrary to me. Just for the sake of it I suggest an alternative solution:
The four boxes form a table with four rows and four columns. In each column, label the numbers like this: A B C D Then (A+B+C+D) mod 3 = 0
Now that I think about it, using the same argument, even
would be a solution. Depending on your creativity, it appears to me that any given number might be a solution to this problem.