Against 4♠, West leads the ♠A and continues with another trump, East following. You are South: what now?
You received a fortunate lead, but you’ll need more good luck to make this contract. If you draw the last trump, you’ll still have to lose a heart and two diamonds for down one. Is there any hope?
Yes: Play four rounds of clubs, intending to pitch one of the hearts from the board. Then you can play the ♥A and ruff a heart with that valuable remaining trump in dummy.
For this to work, however, you’ll need the defender with the last trump to also hold four clubs. On top of that, there’s the small matter of getting back to your hand to draw the remaining enemy trump. You’ll need to be able to ruff the third round of diamonds with a low trump without West overruffing. In other words, if West has the last trump, he must have at least three diamonds.