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

Hence all other candidates can be deleted from these four Cells.

In the example above candidates 2, 3, 8 and 9 are only allowed in cells G4, G5, J4 and J5 of Square "8". Four candidates for four cells; no other candidate is possible in these cells. Candidate 1 can be deleted from J5 and candidate 4 from G5 and J5.

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