Find the number of ways of distributing 4 different balls among 4 identical boxes.

0 + 1 + 7 + 6 + 1 = 15 ways - with no restrictions. This is Bell's 4th number, which is the sum of Stirling Numbers of the 2nd Kind(4, 0), (4,1), (4,2), (4,3) and (4,4).

Thanks a lot man!