Fours are checks — but a check telegraphed a move early can be cancelled outright. Read the enemy position and ask: which single point would make TWO white threats at once? Here white's capped row wants one more stone to become a four, and white's column pair wants the very same point to become a live three — a 4-3, the strongest shape in the game, one move from existing. Occupy the square before the check is born.
Solve the remaining tasks to finish this lesson.