Put the digits 1-3 (later 1-4) in some of the squares so that each row and column contains exactly one of each digit and two blank squares. If two digits are vertically adjacent, the one on top must be larger.

1. | 2. | 3. | 4. | 5. |

6. | 7. | 8. | 9. | 10. |

11. | 12. | 13. | 14. |

15. | 16. | 17. | 18. |

The solutions are here.