You are East on lead in the above position and you decide that to beat declarer's three notrump contract you need to take four spade tricks, which spade do you play?
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You need to switch to the nine of spades, if partner has AQ8x or AJ8x then when declarer covers the nine partner can now win and return a low spade to your king. You can then play the four through declarer's Jx or Qx to pick up the suit. If you start with the four, then declarer can block the suit by not covering the third round.