Extension of XYZ-Wing to four cells; Z is eliminated from cells seeing all four.
Four cells together share four candidates. A specific digit Z appears in every possible configuration of those cells. Any cell seeing all four of these cells can have Z eliminated.
Very difficult to spot manually; computer assistance is usually needed.
| 5 | 1 | 3 | 2 | 9 | 8 | 4 | 7 | 6 |
| 2 | 4 | 6 | 17 | 13 | 37 | 5 | 8 | 9 |
| 7 | 8 | 9 | 46 | 5 | 46 | 1 | 2 | 3 |
| 1 | 6 | 2 | 48 | 348 | 34 | 9 | 5 | 7 |
| 3 | 5 | 4 | 9 | 7 | 2 | 6 | 1 | 8 |
89 | 79 | 78 | 5 | 6 | 1 | 2 | 3 | 4 |
4689 | 3 | 15 | 1468 | 48 | 4569 | 7 | 49 | 2 |
46 | 2 | 17 | 3 | 14 | 4679 | 8 | 49 | 5 |
489 | 79 | 578 | 478 | 2 | 459 | 3 | 6 | 1 |