What is Naked Quad? How to Solve Sudoku

What it is

Like Naked Pairs and Triples, but with four cells. Together the four cells use exactly four digits.

Those four digits are used up by the four cells, so remove them from the rest of the group.

How to spot it

When a group has four cells with few candidates, check whether together they use only four digits.

Example

Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.

In box 3, R1C7, R2C7, R3C7, and R3C9 can only hold 6, 7, 8, and 9. That's 4 cells for 4 digits, so 6, 7, 8, and 9 are used up here (even if we don't know which goes where yet). So the other cells of box 3 (R1C8, R1C9, R2C8, and R2C9) can't be 6, 7, 8, and 9.