If four Cells in a region (Row, Column or Square) contain exactly the same four candidates or only subsets of these four candidates, then one of these candidates is the solution for the first of these Cells, a second one is the solution for the second of these Cells, a third one is the solution for the third of these Cells and the last candidate is the solution for the fourth Cell.

Hence none of these four candidates can be the solution in any other Cell of that region; these four candidates can be deleted from the other Cells of that region.

In the example above, cells A2, B2, C2 and C3 form a Naked Quad in Square "1": four cells with exactly four possible candidates. Each of these candidates must be present in one of these cells. Hence 1 may not be the solution in A1 nor in C1; 3 may not be the solution in B1, B3 nor in C1; 4 may not be the solution in C1; finally, 5 may not be the solution in B3.

You can practice this strategy by installing the free SudokuCoach application on your Android™ device.