Hard Used from hard puzzles up.
Hidden Quad
Four digits, four cells
Four digits that can only go in the same four cells of a unit will fill those cells, one each.
Those cells are then taken: remove every other candidate from them. Like the Naked Quad, it is needed only in a unit with eight or nine empty cells.
What to look for
Find four digits confined to the same four cells of one unit.
Example
- The cells the technique relies on
- Cells where it removes candidates, struck out in red
Practise it
In the app, Learn › Hidden Quad sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.