Naked Quad
Advanced Removes candidates Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
Four cells in a unit share only four candidate digits between them. Those four digits are reserved for those four cells and can be erased from the rest of the unit — the four-cell version of a naked pair or triple.
How to spot it
Four cells in a unit sharing just four candidate digits between them.
What you can conclude
Erase those four digits from every other cell in the unit.
Worked example
Four cells sharing only {1,3,6,9} reserve those digits; remove 1, 3, 6 and 9 from the rest of the unit.