Most of us treat a four no trump bid as always, or almost always, Blackwood. Experts have some sophisticated agreements and the four no trump bid in this auction was quantitative. South accepted the invitation.
South won the opening club lead with dummy’s ace, led a diamond to his ace, and a heart to dummy’s queen. Declarer had 11 top tricks when that won and a twelfth could come from diamonds or spades. A diamond to the queen revealed the foul diamond position and spades became the key suit. West’s singleton diamond made him a favourite to have the spade length. All would be well if the spade honors were split, but good timing would be needed should West have both honors. South cashed three club winners leaving this position:
The king of diamonds was cashed and West could not defend the position. A spade discard and South would just duck a spade, so West shed the jack of hearts. South cashed the ace of hearts and ducked a spade into West. West had to return a spade and South had his slam. Well done!