Step 23 · Master
What is Naked Quad? How to Solve Killer Sudoku
When four cells in a group have only four different candidates between them, you can remove those four digits from the other cells of that group.
What it is
Like Naked Pairs and Triples, but with four cells. Together the four cells use exactly four digits.
Those four digits are used up by the four cells, so remove them from the rest of the group.
How to spot it
When a group has four cells with few candidates, check whether together they use only four digits.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In box 3, R1C8, R1C9, R2C7, and R3C7 can only hold 2, 3, 5, and 6. That's 4 cells for 4 digits, so 2, 3, 5, and 6 are used up here (even if we don't know which goes where yet). So the other cells of box 3 (R2C9 and R3C8) can't be 2, 3, 5, and 6.