1: There are six ways to place each ball, * * * 6 4. 2. Select four boxes to put four balls in two steps, count C (6,4), then arrange the four balls into A (4,4), and then * * * C (6,4) * A (4,4). 3: Select the boxes first, then group the balls, and finally arrange them, * * * c (6,2) * c (4,3) * a (2,2). 4. Classification: 1 one ball in the box, * * C (4, 1) * (5 3), 1 two balls in the box, * * c (4,2) * (52), 1 three balls in the box. Another solution, if it is not empty, has 6 4 kinds, and then subtract the empty case, 5 4, that is, * * * has (6 4-5 4) kinds.