Should White play at , the snapback will destroy Black's chance to make two eyes and the whole group dies.[1]
If Black plays atari at , White captures three stones with
, which also destroys any chance of a second eye.
[1]
Anonymous: Question, given that you cannot take a stone that has just captured one of yours is not move 5 possible since 4 has just taken?
Answer: The rule is that if a single stone captures a single stone, that stone may not be taken back immediately. In the diagram 3 stones take a single stone at move 4, so those three stones may be captured on the next turn.