Quads

Quads are the four-cell version of pairs and triples.

Naked Quad

If four cells in one house contain only the same four candidates, remove those digits from the rest of the house.

In the example above 3, 4, 5 and 6 form a naked quad in R7C7, R8C7, R9C7 and R9C8. Remove the red candidates from the other cells of box 9.

Hidden Quad

If four digits can only appear in the same four cells of a house, remove all other candidates from those cells.

In the example above 1, 3, 5 and 8 form a hidden quad in column 5. Remove the other candidates in R2C5, R3C5, R8C5 and R9C5 (red).