Four in a row problem #525

The task

To move

Dark

Difficulty

Hard

Goal

win in 4

22 discs are already on the grid. Win in four: the threats have to be built in the right order so that every reply is forced.

What to look for

• The key move threatens nothing at first sight; its point comes later.
• The winning line runs along a row.

How the answer is checked

There is exactly one first move that forces the result; any other first move allows the defence to escape. Moves that keep the forced result are accepted later in the line, and a move that loses it is taken back with a short note.