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Hidden Quads

Points.  A hidden quad is formed when all candidates of four digits in a house are restricted to only four cells. Not all cells must contain all 4 digits but together the four cells must contain all 4. All candidates of other digits can be removed from the four cells.

 

 

Hidden Quad, Row.  In row 4, the digits 3, 6,  9, 8 are all contained in only four cells. Therefore no other digits can be present in these four cells and the red candidates can be removed.

 

Hidden Quad, Column.  In column 4, digits 1, 4, 5, and 8 are contained in only four cells. Therefore no other digits can be present in these four cells and the red candidates can be removed.