A Naked Quad is four cells that contain a combination of the same four candidates and are part of the same group. Any other cells in the same group cannot contain the four candidates as those candidates can only be in the Naked Quad cells.
If one of the Naked Quads cells takes a given value, then the other cells in the Naked Quad MUST take the other three values and they become a Naked Triple.
All four candidate do not have to appear in all four cells, but they must not form a Naked Triple.
2, 6, 8 and 9 form the naked quad in Box 1.
That means that candidates 2, 6, 8 and 9 can be removed from cells [1,1], [1,3], [2,1] and [2,3].
3, 5, 8 and 9 form the naked quad in Row 4.
That means that candidates 3, 5, 8 and 9 can be removed from cells [4,4],[4,5] and [4,6].
3, 4, 8 and 9 form the naked quad in Column 2.
That means that candidates 3, 4, 8 and 9 can be removed from cells [1,2],[2,2] and [3,2].
If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.Sample Puzzle 1 Sample Puzzle 2 Sample Puzzle 3