On a club lead from East, North declares the contract of 4Spades. Plan play.
Let us say you quickly take a Heart discard on the king of clubs. Next, what is the first order of the day? A surprising number of declarers played a spade up to the King and a spade back, and now they were short entry and timing to take two diamond finesses. Clearly, there is no rush to pull trumps. If the D finesse is onside, you have ten guaranteed tricks 5 trump, 3 diamonds, 2 clubs. It is essential to take an immediate diamond finesse. Next, try the trumps, they fail but, in with the spade ace, take the second diamond finesse
If instead you play spades, you will lose two hearts and be stuck in hand to lose a diamond along with the trump trick to be lost, (defense will force you in clubs to lead from your hand)