Hidden Quad
Four digits can only go in the same four cells, so every other candidate leaves those cells.
How it works on this board
3, 4, 6 and 9 can appear only in the highlighted set of cells
2
7
5
4
9
6
2
3
9
6
2
9
3
7
2
5
1
6
2
4
5
8
3
3
2
2
8
4
5
4
2
8
9
6
7
6
9
5
3
1
8
4
2
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