white to move and win4
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after b7??? the line will be
b7 BXb7, aXb7 QXb7, black is back to the game
but Qb2 not Qb4 because if it make the line
Qb2! BXa6, BXa6 QXa6, b7! now black can not dtop this pown with out losing his Queen and he can not play Qa1+ because our queen in b2 QXa1
if the queen in b4 squar it cannot prevent mate in a1 ....... Qa1+#
that is whay Qb2 is a very good move