Naked Quad
Four cells whose combined candidate union has exactly four digits. Those four cells must collectively hold those four digits — the rest of the unit loses them.
Intermediate sudoku technique
What it is
A naked quad is four cells inside a single unit (row, column, or box) whose combined candidate union is exactly four digits. Each cell may have all four candidates, or just two or three of the four — what matters is the UNION, not whether every cell carries every digit. Because the four digits collectively need four cells and these are exactly the four cells where they can fit, every OTHER empty cell in the unit must drop those four digits as candidates. Naked quads are the heaviest 'naked' pattern; beyond them you're looking at chain techniques.
When to use it
On hard and expert puzzles when naked triples have stalled. Spot it by listing each unit's empty-cell candidate sets and looking for four cells whose union is exactly four digits.
Worked example
In row 6, four empty cells have candidates {1, 4}, {1, 4, 7}, {4, 7, 9}, {1, 9}. The union is {1, 4, 7, 9} — exactly four digits across four cells. Those four cells must collectively hold 1, 4, 7, 9 in some order. Every OTHER empty cell in row 6 can drop 1, 4, 7, 9 from its candidates.
Try it
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