4. Six schoolchildren decided to plant 5 trees in the schoolyard. It is known that each tree was planted by a different number of schoolchildren and each schoolchild participated in planting the same number of trees. Could this have happened?
4. Yes, this is possible. Here is an example of distributing trees among schoolchildren:

|  | D1 | D2 | D3 | D4 | D5 |
| :--- | :---: | :---: | :---: | :---: | :---: |
| Schoolchild 1 | + | + |  |  | + |
| Schoolchild 2 |  | + |  | + | + |
| Schoolchild 3 |  |  | + | + | + |
| Schoolchild 4 |  |  | + | + | + |
| Schoolchild 5 |  |  | + | + | + |
| Schoolchild 6 |  |  | + | + | + |