Hidden Quad
Advanced Removes candidates Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
Four digits are confined to the same four cells of a unit. Lock them into those cells and clear every other candidate from them, even though each cell still shows extra pencil marks.
How to spot it
Four digits confined to the same four cells of a unit.
What you can conclude
Remove every other candidate from those four cells.
Worked example
When 2, 4, 6 and 8 can only land in four cells, clear every other candidate from them.