How to solve a hidden quad in Sudoku
A hidden quad is four digits that can only go in the same four cells of a unit, so those cells can hold nothing else.
How it works
When four digits are confined to the same four cells in a unit, those cells must hold exactly them.
Any other candidates in those four cells can be removed. It is rare, and the same idea as a hidden pair or triple.
How to spot it
In a nearly finished unit, look for four digits that share the same four cells.
Where it fits in Rocksled Sudoku
Puzzles that need a hidden quad are never rated below Expert. Each step of this kind adds 94 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.