Five in a row problem #521

The task

To move

Dark

Difficulty

Hard

Goal

win in 4

26 stones are already on the lattice. Four moves of fours in a row. Find the start that keeps every block forced to the end.

What to look for

• Continuous fours: each move forces an immediate block.
• An open four appears, and it cannot be stopped.
• Every move forces the reply, so the line can be calculated to the end.

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.