X-Wing extended to four rows/columns; extremely rare in practice.
The next step after Swordfish. When a digit in four rows is confined to at most four columns, it can be eliminated from those columns in all other rows.
Extremely rare in practice. The same logic applies column-wise.
| 6 | 12 | 124 | 8 | 24 | 5 | 7 | 3 | 9 |
25 | 3 | 259 | 6 | 7 | 29 | 1 | 4 | 8 |
| 7 | 89 | 489 | 1 | 3 | 49 | 2 | 5 | 6 |
| 1 | 6 | 25 | 25 | 8 | 3 | 9 | 7 | 4 |
| 8 | 4 | 3 | 9 | 6 | 7 | 5 | 2 | 1 |
| 9 | 257 | 257 | 245 | 245 | 1 | 6 | 8 | 3 |
| 3 | 12579 | 12579 | 2457 | 245 | 24 | 8 | 6 | 27 |
| 4 | 27 | 6 | 27 | 1 | 8 | 3 | 9 | 5 |
25 | 2578 | 2578 | 3 | 9 | 6 | 4 | 1 | 27 |