This is white's sharpest weapon against black's freedom. White's row 7 already reaches toward a gap four — one stone completes it, and the completion point is (7,7). But look at black's cluster: (7,7) is exactly where two black threes would be born at once. So the four demands that black block on a forbidden point: block and lose by foul, or refuse and lose by five. When you are white, hunt for four lines whose five points overlap black's hot spots; when you are black, count your own forbidden points BEFORE the four arrives.
Solve the remaining tasks to finish this lesson.