Hidden quad

Hidden quad is the mirror of naked quad. Four digits in a unit all share the same four-cell pool. Even though the cells may currently list more candidates, only those four digits will end up there — and the others can be erased.

Hidden quads are the rarest of the subset family. Most puzzles never need them.

When to look for it

Saved for tough puzzles where simpler techniques have already exhausted themselves and the candidate grid is dense.

How to apply it

1. For each unit, list each digit's candidate cells.
2. Find four digits whose combined candidate cells are exactly the same four cells.
3. Strip every other candidate from those four cells.

Example