Naked Quad
When four cells in a unit share only four candidates.
Explanation
If four empty cells in one row, column, or box contain only the same four possible digits between them, those digits are reserved for the quad and can be removed from the other cells in that unit.
Practice tip
Use the same scan pattern as naked triples, but allow four cells and four combined digits.
Example steps
- Find four cells in one unit with two to four candidates each.
- Check that their combined candidates are exactly four digits.
- Remove those four digits from the other cells in the unit.