A Hidden Quad is four cells that are part of the same group and contain the same four candidates which are not present in any other cells in the group. The four hidden quad cells should contain more than the four candidates. This means that we can remove all other candidates from the Hidden Quad cells other than the hidden candidates.
If the hidden quad candidates only appear four, three or two times in the group and only in these four cells, they must be the solutions for the cells. Therefore all other candidates in the for hidden quad cells can be elliminated.
All 4 candidate do not have to appear in all 4 cells, but they must not appear in any other cells and they must not form a hidden triple.
1, 3, 4 and 7 form the hidden quad in Box 8.
3, 5, 6 and 9 form the hidden quad in Box 1.
1, 2, 3 and 9 form the hidden quad in Box 6.
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