Hidden Quad
Four digits have only four homes. Clear every other note from those homes.
What it is
Stay in one row, column or box. If four digits can each appear only in the same four empty cells, those cells must contain those four digits. Remove every other candidate from those cells. This clears notes; it does not yet decide which digit goes where.
How to spot it
Do not count every note at once. Pick one unit and ask: which four digits only occur in the same four cells? Mark those four homes, then cross out every extra note inside them.
Worked example
One observation per step: find the four digits with only these four homes, then clear the other notes.
Step 1 of 4: Look at one highlighted unit
Stay in this one row or column. Only the four marked note cells matter first.
Why it matters
The pattern is rare, but its purpose is clear: it turns crowded notes in four cells into a smaller, usable set. Then a simpler follow-up such as a single can place a digit.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
How common are hidden quads?
Very rare in normal play. Most puzzles that appear to need one can be solved another way, so treat it as a curiosity unless you are tackling deliberately extreme grids.
Is there an easier equivalent?
Often yes. If a hidden quad exists in a unit with only five empty cells, there is usually a naked single or naked pair that achieves the same thing with far less work.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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