Arrange the first row arbitrarily: 1 2 3 4. Then there is only one option to begin the next row: 3. The remaining row must be 4 1 2. This leaves no options for beginning the third row (1 and 3 are in the same column, and 2 and 4 share the diagonal).
Arrange the first row arbitrarily: 1 2 3 4. Then there is only one option to begin the next row: 3. The remaining row must be 4 1 2. This leaves no options for beginning the third row (1 and 3 are in the same column, and 2 and 4 share the diagonal).
