Hidden Quad

When four cells in a region contain the same four digits (or a subset of those four digits), and these digits appear in no other cell in the region, they form a Hidden Quad.

The four cells must contain these four digits, and all other candidates in these cells can be eliminated.

Example 1

In box 4 (yellow outline), the shaded cells form a Hidden Quad with the digits 5, 6, 8, and 9. These digits appear only in these four cells, making them the solution for the four digits. Consequently, all other digits can be eliminated from these cells. In this case, digits 1 and 4 can be removed from all the Hidden Quad cells.

Example 2

In box 5, the shaded cells form a Hidden Quad with the digits 4, 6, 7, and 8. These digits appear only in these four cells, making them the solution for the four digits. Consequently, all other digits (highlighted in red) can be eliminated from these cells.

Example 3

In Column 4, the shaded cells form a Hidden Quad with the digits 2, 7, 8, and 9. These digits appear only in these cells, making them the solution for these four digits. Consequently, all other digits (highlighted in red) can be eliminated from these cells.

Example 4

Here we extend the strategy into the world of Sudoku variants. The board includes Killer, Thermo, and Palindrome variants in addition to classic Sudoku.

In the outlined Box 3, the shaded cells form a Hidden Quad with the digits 1, 2, 3, and 8. These digits appear only in these cells, making them the solution for these four digits. Consequently, all other digits (highlighted in red) can be eliminated from these cells.