Hard Used from hard puzzles up.
Naked Quad
Four cells, four candidates
Four cells of a unit whose candidates, taken together, are only four digits will hold those four digits, one each.
Each cell need not have all four: 1 2, 2 3, 3 4 and 1 4 make a quad too. The four digits are used up in that unit: remove them from its other cells.
Quads are rare: only a unit with eight or nine empty cells needs one. With fewer, the same shows as a pair or a triple among its other cells.
What to look for
Find four cells of one unit whose candidates together are just four digits.
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
Practise it
In the app, Learn › Naked Quad sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.