What is Hidden Quad? How to Solve Sudoku

What it is

Like Hidden Pairs and Triples, but with four digits. In a group, four digits can only go in the same four cells.

Those cells must hold those digits, so remove all their other candidates.

How to spot it

Collect digits with few spots and look for four that share the same four cells.

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 5, 1, 2, 3, and 8 can only go in the four cells R4C5, R4C6, R5C5, and R6C5. Those cells must hold 1, 2, 3, and 8, so remove their other candidates (4 and 6).