Four digits confined to the same four cells in a unit; very rare.
Extension of Hidden Triple. Extremely rare and very hard to spot manually. The principle is the same: four digits confined to four cells means all other candidates in those cells can be eliminated.
| 5 | 8 | 3 | 4 | 6 | 1 | 2479 | 156 | 1359 |
| 6 | 2 | 9 | 7 | 5 | 8 | 1456 | 2358 | 468 |
| 7 | 4 | 1 | 9 | 3 | 2 | 156 | 468 | 2378 |
| 1 | 6 | 4 | 8 | 9 | 5 | 3 | 2 | 7 |
| 9 | 3 | 7 | 6 | 2 | 4 | 8 | 5 | 1 |
| 2 | 5 | 8 | 3 | 1 | 7 | 4 | 6 | 9 |
| 4 | 9 | 2 | 5 | 7 | 3 | 1 | 8 | 6 |
| 8 | 7 | 6 | 1 | 4 | 9 | 5 | 3 | 2 |
| 3 | 1 | 5 | 2 | 8 | 6 | 9 | 7 | 4 |
Box 3: Digits 2, 3, 7, and 9 only appear in R1C7, R1C9, R3C7, and R3C8 → Remove all other candidates from those four cells.
This 9×9 puzzle is solver-verified to require this technique on its solution path.