Four is alive
Two four-point shapes are unconditionally alive: the straight four and the bent four. Whatever the attacker tries inside, the defender answers in the remaining space and it still divides into two eyes — try the elbow here and watch Black reply. But count the shape before relaxing: the T-four and the square four are dead, so four points alone is not a certificate of life. A live four-space shape is a quiet gift to your whole game — it needs no defensive move, and in the endgame (Chapter 13) the move you never had to spend is pure profit. This bent four is alive as it stands. White to play: make the strongest killing attempt and see why it fails.