Life In 19x19

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[go]$$Wc diagram 0
$$ | . . . . . . . .
$$ | . O O . . . . .
$$ | . X O . . . . .
$$ | . X X O O . . .
$$ | . 4 . X O . . .
$$ | . O . X X O O .
$$ | . O X a 1 X O .
$$ | 3 X . . 2 X O .
$$ | . . b . . . . .
$$ +-----------------[/go]

This position comes from a problem in Davis' tesuji book ( prob 8 p 137 ).
Davis advises black to compromise with B2@a otherwise it is ko after W3 and B4@b.
This makes me suspect that after B4 as in the diagram it is ko or worse for black.
I tried to figure out why.

Here some diagrams, please correct me if I'm wrong.


edit: BTW: the main problem states that w is supposed to make a sacrifice. It turns out that if B is clever he does not accept the sacrifice but compromises. So the sacrifice is virtual.